sigwait: Do not fail with EINTR and return error code [BZ #22478]

Since

commit 8b0e795a
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Wed Nov 1 11:49:05 2017 -0200

    Simplify Linux sig{timed}wait{info} implementations

sigwait can fail with EINTR.  Applications do not expect that, and the
error code is not documented in POSIX or the manual pages.

This commit restores the previous behavior by retrying the system call
on EINTR.  It also returns the error code, not -1, on the remaing
errors.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>