pgsql: CREATE DOMAIN ...

Log Message:
-----------
CREATE DOMAIN ... DEFAULT NULL failed because gram.y special-cases DEFAULT
NULL and DefineDomain didn't.  Bug goes all the way back to original coding
of domains.  Per bug #3396 from Sergey Burladyan.

Modified Files:
--------------
    pgsql/src/backend/commands:
        typecmds.c (r1.105 -> r1.106)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c.diff?r1=1.105&r2=1.106)