parse-options: export opterr, optbug

opterror and optbug functions are used by some of parsing routines
in parse-options.c to report errors and bugs respectively.

Export these functions to allow more custom parsing routines to use
them in a uniform way.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -165,6 +165,8 @@ extern NORETURN void usage_msg_opt(const char *msg,
 				   const char * const *usagestr,
 				   const struct option *options);
 
+extern int optbug(const struct option *opt, const char *reason);
+extern int opterror(const struct option *opt, const char *reason, int flags);
 /*----- incremental advanced APIs -----*/
 
 enum {