Fix behaviour for search-toggle-regex

text_typeahead_handler: Document that passing -2 for action_id will cause
a search without error reporting.  This behaviour is unintentionally the
current behaviour of text_typeahead_handler, but now it is documented so
that it can be used.

input_line_event_handler: When rewinding, pass -2 for the action_id
parameter to the handler instead of passing again whatever action led to
the rewinding.

The old behavior of input_line_event_handler was particularly problematic
with the search-toggle-regex action and the text_typeahead_handler handler:
input_line_event_handler would call the handler with
ACT_EDIT_SEARCH_TOGGLE_REGEX, and the handler would toggle the setting and
perform the search again; then if the search string no longer matched
anything, the handler would return INPUT_LINE_REWIND to
input_line_event_handler, which would rewind and call the handler with
ACT_EDIT_SEARCH_TOGGLE_REGEX again, thus toggling the option back to the
original setting.

With the new behaviour, input_line_event_handler will not repeat the same
action when re-invoking the handler; in the above example with
search-toggle-regex, the search string will simply be rewound until it
matches with the new setting.

src/bfu/inpfield.c

@@ -856,6 +856,9 @@ cancel_input_line:
 
 		update_dialog_data(dlg_data);
 
+		/* Set action_id to -2 to signal to the handler that it should
+		 * not report errors or take any action except to search. */
+		action_id = -2;
 		goto send_action_to_handler;
 
 	case INPUT_LINE_PROCEED:

src/viewer/text/search.c

@@ -1378,6 +1378,9 @@ do_typeahead(struct session *ses, struct document_view *doc_view,
 /** @name Typeahead
  * @{ */
 
+/** @a action_id can be a value from enum edit_action, in which case the
+ * approriate action is performed; -1, which indicates to search and report any
+ * errors; or -2, which indicates to search without reporting any errors. */
 static enum input_line_code
 text_typeahead_handler(struct input_line *line, int action_id)
 {