Re: Add support for AT LOCAL

One of the AIX animals gave a strange result here:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hornet&dt=2023-10-15%2011%3A40%3A01

If you ignore the diffs due to change in column width, the interesting
change seems to be:

-  23:59:00-07    | 06:59:00+00    | 06:59:00+00    | 06:59:00+00
-  23:59:59.99-07 | 06:59:59.99+00 | 06:59:59.99+00 | 06:59:59.99+00
+  23:59:00-07    | 4294966103:4294967295:00+00 |
4294966103:4294967295:00+00 | 4294966103:4294967295:00+00
+  23:59:59.99-07 | 4294966103:00:00.01+00      |
4294966103:00:00.01+00      | 4294966103:00:00.01+00

But the other AIX animal 'sungazer' was OK with it.  They're both on
the same AIX7.1 host IIRC, both 64 bit builds, but the former is using
xlc and the latter gcc.  I don't immediately see what would cause that
underflow on that old compiler but not elsewhere.  I have a shell
there (cfarm111) if someone has an idea...