* implement / document changing sort order and threading

- update the protocol a bit (mu4e-proc, mu-cmd-server)
  - provide the user-interface (mu4e-headers.el)
  - document it (mu4e.texi, mu-server.1)
  - some cosmetics (the other changes)

lib/mu-query.cc

@@ -323,7 +323,8 @@ mu_query_run (MuQuery *self, const char* searchexpr, gboolean threads,
 		Xapian::Enquire enq (self->db());
 
 		/* note, when our result will be *threaded*, we sort
-		 * there, and don't let Xapian do any sorting */
+		 * in our threading code (mu-threader etc.), and don't
+		 * let Xapian do any sorting */
 		if (!threads && sortfieldid != MU_MSG_FIELD_ID_NONE)
 			enq.set_sort_by_value ((Xapian::valueno)sortfieldid,
 					       revert ? true : false);