Add DTRACE option for postgresql84-server

PR: ports/150431
Submitted by: rpaulo

Fix compilation with GSSAPI

PR: ports/161786
Submitted by: rea
Reviewed by: mandree, John Marshall <john.marshall@riverwillow.com.au>

Mark IGNORE if DTRACE enabled for FreeBSD < 900021 without userland dtrace

PR: ports/152502
Submitted by: Sevan Janiyan <venture37@geeklan.co.uk>

Approved by: maintainer timeout (girgen, at least 2 months)
Feature safe: yes