Introduce pg_settings_get_flags() to find flags associated to a GUC

Introduce pg_settings_get_flags() to find flags associated to a GUC

The most meaningful flags are shown, which are the ones useful for the
user and for automating and extending the set of tests supported
currently by check_guc.

This script may actually be removed in the future, but we are not
completely sure yet if and how we want to support the remaining sanity
checks performed there, that are now integrated in the main regression
test suite as of this commit.

Thanks also to Peter Eisentraut and Kyotaro Horiguchi for the
discussion.

Bump catalog version.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20211129030833[email protected]