options: Add bundled getopt implementation from FreeBSD

Add a portable getopt() fallback for platforms that lack it (e.g. MSVC).
Based on FreeBSD lib/libc/stdlib/getopt.c (BSD-3-Clause), adapted by
replacing FreeBSD internal headers and _getprogname() with standard C
equivalents. Only compiled when HAVE_GETOPT is not defined.

Signed-off-by: Mingyuan Li <2560359315@qq.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/external/getopt.c

@@ -0,0 +1,134 @@
+/*	$NetBSD: getopt.c,v 1.29 2014/06/05 22:00:22 christos Exp $	*/
+
+/*-
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 1987, 1993, 1994
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * Adapted for libssh: removed FreeBSD internal headers (namespace.h,
+ * un-namespace.h, libc_private.h, unistd.h) and replaced _getprogname()
+ * with nargv[0] for portability on platforms lacking getopt (e.g. MSVC).
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+int opterr = 1,      /* if error message should be printed */
+    optind = 1,      /* index into parent argv vector */
+    optopt,          /* character checked for validity */
+    optreset;        /* reset getopt */
+char *optarg = NULL; /* argument associated with option */
+
+#define BADCH  (int)'?'
+#define BADARG (int)':'
+static char EMSG[] = "";
+/*
+ * getopt --
+ *	Parse argc/argv argument vector.
+ */
+int getopt(int nargc, char *const nargv[], const char *ostr);
+
+int getopt(int nargc, char *const nargv[], const char *ostr)
+{
+    static char *place = EMSG; /* option letter processing */
+    char *oli = NULL;          /* option letter list index */
+
+    if (optreset || *place == 0) { /* update scanning pointer */
+        optreset = 0;
+        if (optind >= nargc || *(place = nargv[optind]) != '-') {
+            /* Argument is absent or is not an option */
+            place = EMSG;
+            return (-1);
+        }
+        place++;
+        optopt = *place++;
+        if (optopt == '-' && *place == 0) {
+            /* "--" => end of options */
+            ++optind;
+            place = EMSG;
+            return (-1);
+        }
+        if (optopt == 0) {
+            /* Solitary '-', treat as a '-' option
+               if the program (eg su) is looking for it. */
+            place = EMSG;
+            if (strchr(ostr, '-') == NULL)
+                return (-1);
+            optopt = '-';
+        }
+    } else
+        optopt = *place++;
+
+    /* See if option letter is one the caller wanted... */
+    if (optopt == ':' || (oli = strchr(ostr, optopt)) == NULL) {
+        if (*place == 0)
+            ++optind;
+        if (opterr && *ostr != ':')
+            (void)
+                fprintf(stderr, "%s: illegal option -- %c\n", nargv[0], optopt);
+        return (BADCH);
+    }
+
+    /* Does this option need an argument? */
+    if (oli[1] != ':') {
+        /* don't need argument */
+        optarg = NULL;
+        if (*place == 0)
+            ++optind;
+    } else {
+        /* Option-argument is either the rest of this argument or the
+           entire next argument. */
+        if (*place)
+            optarg = place;
+        else if (oli[2] == ':')
+            /*
+             * GNU Extension, for optional arguments if the rest of
+             * the argument is empty, we return NULL
+             */
+            optarg = NULL;
+        else if (nargc > ++optind)
+            optarg = nargv[optind];
+        else {
+            /* option-argument absent */
+            place = EMSG;
+            if (*ostr == ':')
+                return (BADARG);
+            if (opterr)
+                (void)fprintf(stderr,
+                              "%s: option requires an argument -- %c\n",
+                              nargv[0],
+                              optopt);
+            return (BADCH);
+        }
+        place = EMSG;
+        ++optind;
+    }
+    return (optopt); /* return option letter */
+}