pgsql: Update Darwin dlopen() support to avoid deprecation warnings with

Log Message:
-----------
Update Darwin dlopen() support to avoid deprecation warnings with latest
Apple developer tools. We now use dlopen directly if available, and fall
back to the older code if not. Chris Campbell

Modified Files:
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pgsql/src/backend/port/dynloader:
darwin.c (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/dynloader/darwin.c.diff?r1=1.10&r2=1.11)