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/fd.c b/src/fd.c
index ea5d683..e8419aa 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -202,7 +202,7 @@
 	fdtab[fd].owner = NULL;
 	fdtab[fd].updated = 0;
 	fdtab[fd].new = 0;
-	fdtab[fd].process_mask = 0;
+	fdtab[fd].thread_mask = 0;
 	if (do_close)
 		close(fd);
 	SPIN_UNLOCK(FD_LOCK, &fdtab[fd].lock);
@@ -245,7 +245,7 @@
 	for (entry = 0; entry < fd_cache_num; ) {
 		fd = fd_cache[entry];
 
-		if (!(fdtab[fd].process_mask & tid_bit))
+		if (!(fdtab[fd].thread_mask & tid_bit))
 			goto next;
 		if (SPIN_TRYLOCK(FD_LOCK, &fdtab[fd].lock))
 			goto next;