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_kqueue.c b/src/ev_kqueue.c
index 2ae2774..00dc965 100644
--- a/src/ev_kqueue.c
+++ b/src/ev_kqueue.c
@@ -122,7 +122,7 @@
 		unsigned int n = 0;
 		fd = kev[count].ident;
 
-		if (!fdtab[fd].owner || !(fdtab[fd].process_mask & tid_bit))
+		if (!fdtab[fd].owner || !(fdtab[fd].thread_mask & tid_bit))
 			continue;
 
 		if (kev[count].filter ==  EVFILT_READ) {