- 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

Remove NetBSD Foundation clauses 3 & 4. They eliminated those clauses
last year.

Add getopt_long_only()

In the comment tell about optreset extension for NetBSD ghetopt_long()

Add optreset to getopt.h too since NetBSD getopt_long() (but not GNU one)
use it too.

Try to better mimic GNU getopt.h which does not assume to make visible
all unistd.h functions, use _GETOPT_DECLARE define for that.

Fix comment: GNU getopt.h to which this header tries to mimic does not declare
getsubopt/optreset

Add getopt_long(3).

Obtained from: NetBSD
Sponsored by: Apple