pgsql: Add: > > o Allow row and record variables to be set to NULL

Log Message:
-----------
Add:

o Allow row and record variables to be set to NULL constants,
and allow NULL tests on such variables

Because a row is not scalar, do not allow assignment
from NULL-valued scalars.

Modified Files:
--------------
pgsql/doc:
TODO (r1.2095 -> r1.2096)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.2095&r2=1.2096)
pgsql/doc/src/FAQ:
TODO.html (r1.596 -> r1.597)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.596&r2=1.597)