Have functions deprecated in 2.1.0 raise a WARNING message (#1994)

Drop use of some deprecated functions by other functions
(as exposed by testsuite).

Add a _postgis_deprecate service function for the message, so we
can change from WARNING to NOTICE or we can tweak the message,
in a central place.

git-svn-id: http://svn.osgeo.org/postgis/trunk@11191 b70326c6-7e19-0410-871a-916f4a2858ee