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/include/types/fd.h b/include/types/fd.h
index e196aec..2da6599 100644
--- a/include/types/fd.h
+++ b/include/types/fd.h
@@ -94,7 +94,7 @@
 struct fdtab {
 	void (*iocb)(int fd);                /* I/O handler */
 	void *owner;                         /* the connection or listener associated with this fd, NULL if closed */
-	unsigned long process_mask;          /* mask of thread IDs authorized to process the task */
+	unsigned long thread_mask;           /* mask of thread IDs authorized to process the task */
 #ifdef USE_THREAD
 	HA_SPINLOCK_T lock;
 #endif