Revert "include/nuttx/sched.h: Add storage for a previous signal mask. arch/: In all syscall implementations, block all signals before dispatching a system call; resotre signal mask when the system call returns."

Using the sigprocmask() for this purpose has too many side-effects.

This reverts commit 344f7bc9f6.

net/netlink/netlink_conn.c

@@ -127,8 +127,8 @@ static void netlink_notify_waiters(FAR struct netlink_conn_s *conn)
   int i;
 
   /* Notify every pending thread.  Lock the scheduler while we do this so
-   * that there is no thrashing:  All waiters will be restarted, but only
-   * the highest priority waiter will get to run and it will receive the
+   * there there is no thrashing:  All waiters will be restarted, but only
+   * the highest priority waiter will get to run and will receive the
    * response.
    */