Sanitizes given parameters and searches for PMs based on those parameters

• Currently does not support searching the outbox
• Currently does not support sorting by anything other than most recent or least recent
• Does not search if it finds more than 500 users matching the specified "user" criteria (hard-coded limit)
• Excludes any phrases preceded by a "-" (ie -"some phrase")
• Excludes any words preceded by a "-" (ie -word)
• Allows you to return to search later (implodes search parameters array with |"| then applies base64_encode())
• Returns to MessageSearch() with encoded parameters in $_REQUEST['params'] if it runs into an error
• Part of the "search" PM area
• Uses the search_results sub-template