Bump __FreeBSD_version to 800500 which should have happened already

Bump __FreeBSD_version to 800500 which should have happened already
after r198460 but was missed.

Note: that 800108 should have been 800501 with that but as there is no
functional problem here, it'll just stay as is. [1]

This will make pkg_add -r use packages-8-stable for stable/8 rather
than packages-8.0-release.

Reported by:	Paride Legovini (pl ninthfloor.org) on stable@,
		(pluknet gmail.com), jhb
Discussed with:	rwatson [1]

git-svn-id: svn://svn.freebsd.org/base/stable/8@201749 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f