This should have been done in the initial commit that made
unix_socket_directories a list as of c9b0cbe9.  This change allows to
support correctly the case of ALTER SYSTEM, where it is possible to
specify multiple paths as a list, like the following pattern where
flattening is applied to each item:
ALTER SYSTEM SET unix_socket_directories = '/path1', '/path2';

Any parameters specified in postgresql.conf are parsed the same way, so
there is no compatibility change.  pg_dump has a hardcoded list of
parameters marked with GUC_LIST_QUOTE, that gets its routine update.
These are reordered alphabetically for clarity.

Author: Ian Lawrence Barwick
Reviewed-by: Peter Eisentraunt, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/CAB8KJ=iMOtNY6_sUwV=LQVCJ2zgYHBDyNzVfvE5GN3WQ3v9kQg@mail.gmail.com