back-patching locale environment fix

I'm looking at back-patching these fixes into the release branches:
http://archives.postgresql.org/pgsql-committers/2005-12/msg00479.php
http://archives.postgresql.org/pgsql-committers/2006-01/msg00017.php

This is mostly pretty straightforward, except that the patch depends on
having unsetenv(), which we don't currently have port support for before
8.0.  So I'll need to stick src/port/unsetenv.c into the 7.4 and 7.3
branches and add the appropriate configure tests.

This makes the patch a bit larger than I would like, but I don't see a
whole lot of alternative: plperl is simply hazardous to the health of
your database without this change.

Comments, objections?
        regards, tom lane