CLEANUP: threads: rename process_mask to thread_mask

It was a leftover from the last cleaning session; this mask applies
to threads and calling it process_mask is a bit confusing. It's the
same in fd, task and applets.
diff --git a/src/ev_epoll.c b/src/ev_epoll.c
index d26c3a2..65e5164 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -136,7 +136,7 @@
 		unsigned int e = epoll_events[count].events;
 		fd = epoll_events[count].data.fd;
 
-		if (!fdtab[fd].owner || !(fdtab[fd].process_mask & tid_bit))
+		if (!fdtab[fd].owner || !(fdtab[fd].thread_mask & tid_bit))
 			continue;
 
 		/* it looks complicated but gcc can optimize it away when constants