linuxthreads/signal: improve sigaction behavior

Setting signal handler in the kernel and then updating sighandler[sig]
results in a crash if a signal which handler is being changed from
SIG_DFL to a non-default was pending. Improve the race a little and
update the sighandler[sig] before the sigaction syscall. It doesn't
eliminate the race entirely, but fixes this particular failing case.

E.g. this fixes the 100% reproducible segfault in the busybox hush shell
built with FEATURE_EDITING_WINCH on ssh client's terminal window resize,
but in that case there's one more even bigger issue: busybox calls
sigaction with both old and new signal pointers pointing to the same
structure instance, as a result act->sa_handler after the sigaction
syscall is not what the user requested, but the previous handler.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

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