hurd: Fix pselect atomicity

hurd: Fix pselect atomicity

In case the signal arrives before the __mach_msg call, we need to catch
between the sigprocmask call and the __mach_msg call.  Let's just reuse
the support for sigsuspend to make the signal send a message that
our __mach_msg call will just receive.

* hurd/hurdselect.c (_hurd_select): Add sigport and ss variables. When
sigmask is not NULL, create a sigport port and register as
ss->suspended.  Add it to the portset.  When we receive a message on it,
set error to EINTR.  Clean up sigport and portset appropriately.

* hurd/hurdsig.c (wake_sigsuspend): Note that pselect also uses it.