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/task.c b/src/task.c
index 7623da9..1768e07 100644
--- a/src/task.c
+++ b/src/task.c
@@ -229,7 +229,7 @@
 		while (local_tasks_count < 16) {
 			t = eb32_entry(rq_next, struct task, rq);
 			rq_next = eb32_next(rq_next);
-			if (t->process_mask & tid_bit) {
+			if (t->thread_mask & tid_bit) {
 				/* detach the task from the queue */
 				__task_unlink_rq(t);
 				t->state |= TASK_RUNNING;