Use non-deprecated APIs for dynloader/darwin.c

Re: Darwin stuff is getting deprecated

http://archives.postgresql.org/pgsql-hackers/2006-09/msg02238.php

Mac OS X has included dlopen() and friends etc since Mac OS X 10.3.
From the dlopen(3) man page:

Mac OS X 10.3 incorporated the dlcompat package written by
Jorge Acereda
<jacereda(at)users(dot)sourceforge(dot)net> and Peter O'Gorman <ogor-
man(at)users(dot)sourceforge(dot)net>.

In Mac OS X 10.4, dlopen was rewritten to be a native part of
dyld.

The attached patch switches to using those APIs in src/backend/port/
dynloader/darwin.c (and passes "make check").

(1) This change requires Mac OS X 10.3 and later. Do we currently
support PostgreSQL on older versions of Mac OS X?

(2) The pg_* functions are such trivial wrappers for the OS's APIs --
should they be turned into macros in dynloader/darwin.h and we can
just delete the .c file?

(3) If we need to continue supporting Mac OS X 10.2 and earlier, I
guess we'll need to add a configure test for the presence of dlopen
()? Is that a check that should only be run on Darwin, or all platforms?

Thanks!

- Chris