misc: Add function to check username syntax

Malicious code can be injected using the username with metacharacters,
therefore the username must be validated before using it with any %u.

Signed-off-by: Norbert Pocs <norbertpocs0@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/misc.h

@@ -123,6 +123,7 @@ ssize_t ssh_readn(int fd, void *buf, size_t nbytes);
 ssize_t ssh_writen(int fd, const void *buf, size_t nbytes);
 
 int ssh_check_hostname_syntax(const char *hostname);
+int ssh_check_username_syntax(const char *username);
 
 #ifdef __cplusplus
 }