Porting PostgreSQL to DragonFly BSD

From what I have checked, all work on making postgresql compile on new
platforms should be posted to this list, so here it goes.

The attached patch (also available at
http://www.rumko.net/0001-DragonFly-BSD-support.patch ) applies cleanly to the
master branch and can be cherry-picked to REL9_0_STABLE branch without
conflicts.

It's based on postgres' FreeBSD support with minimal changes (a few
freebsd->dragonfly renames, removed mips support since dragonfly does not
support it and it also includes a patch to properly define the linker on dfly
as per PR pkg/44617 in http://www.netbsd.org/support/query-pr.html gnats
database).
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Regards,
Rumko