Mark pg_config() stable rather than immutable

pg_config() has been marked immutable since its inception. As part of a
larger discussion around the definition of immutable versus stable and
related implications for marking functions parallel safe raised by
Andres, the consensus was clearly that pg_config() is stable, since
it could possibly change output even for the same minor version with
a recompile or installation of a new binary. So mark it stable.

Theoretically this could/should be backpatched, but it was deemed to be not
worth the effort since in practice this is very unlikely to cause problems
in the real world.

Discussion: https://postgr.es/m/20181126234521.rh3grz7aavx2ubjv@alap3.anarazel.de

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