emacs: Fix search filtering of a global search ("*")

With the recent addition of "*" being a special case for a search
matching all messages, we have to take care when doing a filter
operation. In this case it's not legal to simply append and get:

	* and <some-new-search-terms>

Instead we carefully construct a new search string of only:

	<some-new-search-terms>

This could all be avoided if we had a parser that could understand
"*" with the meaning we want.