When stable/10 was branched from head/, __FreeBSD_version was bumped

When stable/10 was branched from head/, __FreeBSD_version was bumped
to 1000500 from 1000055 when it should not have been bumped yet.

At the risk of having non-standard '5XX' __FreeBSD_version suffix
in a -RELEASE, bump __FreeBSD_version in releng/10.0 from 1000100
to 1000510 to prevent the value from going backwards as part of the
stable/10 -> releng/10.0 branch.

A commit to bump __FreeBSD_version in stable/10 will follow.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/releng/10.0@259067 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f