Simplify main waiting loop of the archiver process

Simplify main waiting loop of the archiver process

As coded, the timeout given to WaitLatch() was always equal to
PGARCH_AUTOWAKE_INTERVAL, as time() was called two times repeatedly.
This simplification could have been done in d75288f.

While on it, this adjusts a comment in pgarch.c to describe the archiver
in a more neutral way.

Author: Sravan Kumar, Nathan Bossart
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CA+=NbjjqYE9-Lnw7H7DAiS5jebmoMikwZQb_sBP7kgBCn9q6Hg@mail.gmail.com