Fix a bunch of problems with domains by making them use special input functions

that apply the necessary domain constraint checks immediately.  This fixes
cases where domain constraints went unchecked for statement parameters,
PL function local variables and results, etc.  We can also eliminate existing
special cases for domains in places that had gotten it right, eg COPY.

Also, allow domains over domains (base of a domain is another domain type).
This almost worked before, but was disallowed because the original patch
hadn't gotten it quite right.