pgsql: Create a function quote_nullable(), which works the same as

Log Message:
-----------
Create a function quote_nullable(), which works the same as quote_literal()
except that it returns the string 'NULL', rather than a SQL null, when called
with a null argument. This is often a much more useful behavior for
constructing dynamic queries. Add more discussion to the documentation
about how to use these functions.

Brendan Jurd

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.424 -> r1.425)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.424&r2=1.425)
plpgsql.sgml (r1.123 -> r1.124)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml?r1=1.123&r2=1.124)
pgsql/src/backend/utils/adt:
quote.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/quote.c?r1=1.23&r2=1.24)
pgsql/src/include/catalog:
catversion.h (r1.443 -> r1.444)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.443&r2=1.444)
pg_proc.h (r1.483 -> r1.484)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.483&r2=1.484)
pgsql/src/include/utils:
builtins.h (r1.308 -> r1.309)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.308&r2=1.309)