Rename notmuch_query_search to notmuch_query_search_messages

Along with renaming notmuch_results_t to notmuch_message_results_t.
The new type is quite a mouthful, but I don't expect it to be
used much other than the for-loop idiom in the documentation,
(which does at least fit nicely within 80 columns).

This is all in preparation for the addition of a new
notmuch_query_search_threads of course.