cmdutils: split per-option code out of parse_options().

This allows options like -target, which are just shortcuts for other
options, to work without dummy function for all options they invoke.

cmdutils.h

@@ -151,6 +151,13 @@ void show_help_options(const OptionDef *options, const char *msg, int mask, int
 void parse_options(void *optctx, int argc, char **argv, const OptionDef *options,
                    void (* parse_arg_function)(void *optctx, const char*));
 
+/**
+ * Parse one given option.
+ *
+ * @return on success 1 if arg was consumed, 0 otherwise; negative number on error
+ */
+int parse_option(void *optctx, const char *opt, const char *arg, const OptionDef *options);
+
 /**
  * Check if the given stream matches a stream specifier.
  *