Improve readability of code PROCESS_MAIN in vacuum_rel()

Improve readability of code PROCESS_MAIN in vacuum_rel()

4211fbd has been handling PROCESS_MAIN in vacuum_rel() with an "if/else
if" structure to avoid an extra level of indentation, but this has been
found as being rather parse to read.  This commit updates the code so as
we check for PROCESS_MAIN in a single place and then handle its
subpaths, FULL or non-FULL vacuums.  Some comments are added to make
that clearer for the reader.

Reported-by: Melanie Plageman
Author: Nathan Bossart
Reviewed-by: Michael Paquier, Melanie Plageman
Discussion: https://postgr.es/m/20230306194009.5cn6sp3wjotd36nu@liskov