pgsql: Implement md5(bytea), update regression tests and documentation.

Log Message:
-----------
Implement md5(bytea), update regression tests and documentation. Patch
from Abhijit Menon-Sen, minor editorialization from Neil Conway. Also,
improve md5(text) to allocate a constant-sized buffer on the stack
rather than via palloc.

Catalog version bumped.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.247 -> r1.248)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.247&r2=1.248)
pgsql/src/backend/utils/adt:
varlena.c (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.120&r2=1.121)
pgsql/src/include/catalog:
catversion.h (r1.268 -> r1.269)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.268&r2=1.269)
pg_proc.h (r1.362 -> r1.363)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.362&r2=1.363)
pgsql/src/include/utils:
builtins.h (r1.255 -> r1.256)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h.diff?r1=1.255&r2=1.256)
pgsql/src/test/regress/expected:
strings.out (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/strings.out.diff?r1=1.24&r2=1.25)
pgsql/src/test/regress/sql:
strings.sql (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/strings.sql.diff?r1=1.15&r2=1.16)