Removing dead support for pre-POSIX signals

A question in another project's mailing list prompted me to investigate
whether our support for non-POSIX signals is actually still of any value.
I grepped through the buildfarm server's configure-stage reports, and
found that:

1. No buildfarm member in the available history (going back to 2012-01-01)
has ever reported not having the POSIX signal interface, nor sig_atomic_t.
(We don't run the POSIX-signals check on Windows systems, though.)

2. No non-Windows member has ever reported not having sigprocmask nor
sigsetjmp, save for two one-time failures that look to be due to configure
failing for being out of disk space.

3. The check for sys_siglist is still of use, which is unsurprising
because it is not required by POSIX.

The POSIX signal APIs, including sigprocmask and sigsetjmp, are required
by Single Unix Spec v2, which is what we've usually considered to be our
minimum baseline for supported Unix-ish systems.

I think we should rip out the configure checks for HAVE_POSIX_SIGNALS,
HAVE_SIGPROCMASK, HAVE_SIGSETJMP, and probably HAVE_SIG_ATOMIC_T, as well
as the C code that tries to make up for not having these things (on
non-Windows systems that is).  It's not being exercised and it's fair to
doubt that those code paths would even work reliably anymore.  For
instance, it seems likely that our current latch code has never been run
on a system without these APIs, and even more likely that Andres' recent
fooling around with signal handling (eg commit 6753333f5) has never been
checked on such a system.

HAVE_SIG_ATOMIC_T is a debatable case, in that the only thing we're
doing with it is c.h's

/* sig_atomic_t is required by ANSI C, but may be missing on old platforms */
#ifndef HAVE_SIG_ATOMIC_T
typedef int sig_atomic_t;
#endif

which should be safe enough (if int isn't atomically stored/fetched we
already have big problems elsewhere).  Still, the configure test for it
appears to be a complete waste of cycles.

Comments?
        regards, tom lane