Enable ssh agent authentication on Windows

Windows has supported unix domain sockets for a couple of years
now; see this article for more information about that:
<https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/>

This commit allows libssh to consider using agent authentication
on Windows systems.

It is mostly removing `#ifndef _WIN32` that prevented the unix
domain socket code from being compiled in, and adjusting the use
of `read(2)` and `write(2)` to `recv(2)` and `send(2)`, as the former
functions are not compatible with sockets on Windows systems.

For mingw systems, afunix.h isn't available so we use the
technique as was used to resolve building with mingw as used
by the curl project in: https://github.com/curl/curl/pull/5170

Signed-off-by: Wez Furlong <wez@fb.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/socket.c

@@ -28,6 +28,15 @@
 #ifdef _WIN32
 #include <winsock2.h>
 #include <ws2tcpip.h>
+#ifndef UNIX_PATH_MAX
+ /* Inlining the key portions of afunix.h in Windows 10 SDK;
+  * that header isn't available in the mingw environment. */
+#define UNIX_PATH_MAX 108
+typedef struct sockaddr_un {
+  ADDRESS_FAMILY sun_family;
+  char sun_path[UNIX_PATH_MAX];
+};
+#endif
 #if _MSC_VER >= 1400
 #include <io.h>
 #undef open
@@ -410,7 +419,6 @@ void ssh_socket_free(ssh_socket s)
     SAFE_FREE(s);
 }
 
-#ifndef _WIN32
 int ssh_socket_unix(ssh_socket s, const char *path)
 {
     struct sockaddr_un sunaddr;
@@ -426,24 +434,25 @@ int ssh_socket_unix(ssh_socket s, const char *path)
         return -1;
     }
 
+#ifndef _WIN32
     if (fcntl(fd, F_SETFD, 1) == -1) {
         ssh_set_error(s->session, SSH_FATAL,
                       "Error from fcntl(fd, F_SETFD, 1): %s",
                       strerror(errno));
-        close(fd);
+        CLOSE_SOCKET(fd);
         return -1;
     }
+#endif
 
     if (connect(fd, (struct sockaddr *) &sunaddr, sizeof(sunaddr)) < 0) {
         ssh_set_error(s->session, SSH_FATAL, "Error from connect(): %s",
                       strerror(errno));
-        close(fd);
+        CLOSE_SOCKET(fd);
         return -1;
     }
     ssh_socket_set_fd(s,fd);
     return 0;
 }
-#endif
 
 /** \internal
  * \brief closes a socket