* descr.h: Define SETXID_BIT and SETXID_BITMASK. Adjust
CANCEL_RESTMASK.
(struct pthread): Move specific_used field to avoid padding.
Add setxid_futex field.
* init.c (sighandler_setxid): Reset setxid flag and release the
setxid futex.
* allocatestack.c (setxid_signal_thread): New function. Broken
out of the bodies of the two loops in __nptl_setxid. For undetached
threads check whether they are exiting and if yes, don't send a signal.
(__nptl_setxid): Simplify loops by using setxid_signal_thread.
* pthread_create.c (start_thread): For undetached threads, check
whether setxid bit is set. If yes, wait until signal has been
processed.