Update devel/{boehm-gc,boehm-gc-threaded,boehm-gc-redirect} to
7.2d, drop some patches that are obsolete, add a patch that makes
PARALLEL_MARK in boehm-gc-threaded work (previously it was a
no-op, as it turned out), update Makefile headers to the new
style.

The changelog for 7.2d is at [1]. There are quite a few fixes
since 7.1, including a fix for the crash in GC_FreeBSDGetDataStart
on startup that prevented a few programs (e.g. lang/stklos) from
working on FreeBSD 9.0.

Note that the threaded version of boehm-gc is know to crash
randomly under FreeBSD, even on it's own test suite. AFAIK, this
has always been the case (I haven't tested 7.2d specifically,
but builds up until 7.2c where definitely affected; see [3] and
followups). The crashes are infrequent though.

The redports logs at [3] (the version tested differs from the
patch I'm submitting only in that PARALLEL_MARK is enabled by
default there, but disabled here -- it's the vendor's default
to disable it, so let's do that too).

Note that boehm-gc contains lots of platform-specific code, so
if the committer would be able to run 'make regression-test' on
ARM or SPARC that would be appreciated.

[1] https://github.com/ivmai/bdwgc/blob/gc7_2d/ChangeLog
[2] http://www.hpl.hp.com/hosted/linux/mail-archives/gc/2012-July/005282.html
[3] https://redports.org/buildarchive/20130210152500-62034/

Fix: Patch attached with submission follows: