- 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

Add bsd.port.options.mk, a stub to include parts of bsd.port.mk that handle
OPTIONS resolving. This will allow us to load bsd.port.mk in port Makefiles in
three steps (options, pre, post), allowing us to manipulate USE_* flags
conditionally on OPTIONS values.

With hat: portmgr
Reviewed by: ru
MFC after: 1 week