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/applet.h b/include/types/applet.h
index b56c563..1c469a5 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -67,7 +67,7 @@
 	                                               if the command is terminated or the session released */
 	int cli_severity_output;        /* used within the cli_io_handler to format severity output of informational feedback */
 	struct buffer_wait buffer_wait; /* position in the list of objects waiting for a buffer */
-	unsigned long process_mask;     /* mask of thread IDs authorized to process the applet */
+	unsigned long thread_mask;      /* mask of thread IDs authorized to process the applet */
 
 	union {
 		struct {
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
diff --git a/include/types/task.h b/include/types/task.h
index 799e6c5..236324d 100644
--- a/include/types/task.h
+++ b/include/types/task.h
@@ -72,7 +72,7 @@
 	void *context;			/* the task's context */
 	struct eb32_node wq;		/* ebtree node used to hold the task in the wait queue */
 	int expire;			/* next expiration date for this task, in ticks */
-	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 */
 };
 
 /*