Allow ^substr, substr=, ~word in mairix searches via the form widget.

Allow ^substr, substr=, ~word in mairix searches via the form widget.

 lisp/net/mairix.el (mairix-replace-invalid-chars): Rename from
 mairix-replace-illegal-chars; all callers changed.  Don't remove
 ^, ~, and = characters: they are meaningful in mairix search
 specs.
 (mairix-widget-create-query): Add usage information about mairix
 search forms: negating words, searching for substrings, etc.