- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

As threatened, drop support for source upgrades from pre-5.3.

Inspired by: obrien

Make sure legacy stuff comes last. Some tools that use
legacy stuff (binutils) depend on this order.

For this to work, provide (and use) specialized versions
of bsd.prog.mk and bsd.lib.mk that include the standard
versions first, then augment CFLAGS, DPADD, LDADD, and
LDFLAGS as necessary, with the legacy stuff.

Tested on: 4.0-RELEASE