Move check_user_shell() to pwutil.c as user_shell_valid()

This will make it possible to support a different backend which may
be used by testsudoers in the future.

plugins/sudoers/pwutil.h

@@ -73,5 +73,6 @@ struct cache_item *sudo_make_gritem(gid_t gid, const char *group);
 struct cache_item *sudo_make_grlist_item(const struct passwd *pw, char * const *groups);
 struct cache_item *sudo_make_gidlist_item(const struct passwd *pw, int ngids, GETGROUPS_T *gids, char * const *gidstrs, unsigned int type);
 struct cache_item *sudo_make_pwitem(uid_t uid, const char *user);
+bool valid_shell(const char *shell);
 
 #endif /* SUDOERS_PWUTIL_H */