nptl: Retry SIGSETXID delivery on EAGAIN in setxid (bug 21108)

[Wilco Dijkstra]

Hi Adhemerval,

>   setxid_signal_thread assumed tgkill could only fail if the target
> thread had not started yet or had already exited, and treated any error
> as "thread is gone".  But SIGSETXID is a realtime signal, and for those
> the tgkill may fail with with EAGAIN once the per-real-user
> RLIMIT_SIGPENDING limit is reached (process-directed kill uses the
> guaranteed delivery override, but a thread-directed tgkill cannot.)

LGTM

Cheers,
Wilco