pgsql: Ignore more environment variables in pg_regress.c

Ignore more environment variables in pg_regress.c

This is similar to the work done in 8279f68 for TestLib.pm, where
environment variables set may cause unwanted failures if using a
temporary installation with pg_regress.  The list of variables reset is
adjusted in each stable branch depending on what is supported.

Comments are added to remember that the lists in TestLib.pm and
pg_regress.c had better be kept in sync.

Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/YMNR9GYDn+fHlMta@paquier.xyz
Backpatch-through: 9.6

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/37d4edef00e40085f31bfd578b07c5e319e9818a

Modified Files
--------------
src/test/perl/TestLib.pm      |  1 +
src/test/regress/pg_regress.c | 27 +++++++++++++++++++++++----
2 files changed, 24 insertions(+), 4 deletions(-)