CVE-2023-6004: misc: Add function to check allowed characters of a hostname

The hostname can be a domain name or an ip address. The colon has to be
allowed because of IPv6 even it is prohibited in domain names.

Signed-off-by: Norbert Pocs <norbertpocs0@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/misc.h

@@ -103,6 +103,9 @@ int ssh_newline_vis(const char *string, char *buf, size_t buf_len);
 int ssh_tmpname(char *name);
 
 char *ssh_strreplace(const char *src, const char *pattern, const char *repl);
+
+int ssh_check_hostname_syntax(const char *hostname);
+
 #ifdef __cplusplus
 }
 #endif