Fix old pg_dump oversight: default values for domains really need to be dumped

Fix old pg_dump oversight: default values for domains really need to be dumped
by decompiling the typdefaultbin expression, not just printing the typdefault
text which may be out-of-date or assume the wrong schema search path.  (It's
the same hazard as for adbin vs adsrc in column defaults.)  The catalogs.sgml
spec for pg_type implies that the correct procedure is to look to
typdefaultbin first and consider typdefault only if typdefaultbin is NULL.
I made dumping of both domains and base types do that, even though in the
current backend code typdefaultbin is always correct for domains and
typdefault for base types --- might as well try to future-proof it a little.
Per bug report from Alexander Galler.