cli: export function for illegal tag checking

This lets us check for forbidden tags consistently across the cli. No
functional changes.

tag-util.c

@@ -31,14 +31,7 @@ line_error (tag_parse_status_t status,
     return status;
 }
 
-/*
- * Test tags for some forbidden cases.
- *
- * return: NULL if OK,
- *	   explanatory message otherwise.
- */
-
-static const char *
+const char *
 illegal_tag (const char *tag, notmuch_bool_t remove)
 {
 

tag-util.h

@@ -89,6 +89,18 @@ tag_parse_status_t
 parse_tag_command_line (void *ctx, int argc, char **argv,
 			char **query_str, tag_op_list_t *ops);
 
+/*
+ * Test tags for some forbidden cases.
+ *
+ * Relax the checks if 'remove' is true to allow removal of previously
+ * added forbidden tags.
+ *
+ * return: NULL if OK,
+ *	   explanatory message otherwise.
+ */
+const char *
+illegal_tag (const char *tag, notmuch_bool_t remove);
+
 /*
  * Create an empty list of tag operations
  *