- 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

POSIXed strtoll() (and ours one too) can set errno to EINVAL, so check
it first.

Approved by: andre

Import of OpenBSD's strtonum(3) which is a nicer version of strtoll(3)
providing proper error checking and other improvements.

Obtained from: OpenBSD
Requested by: flz (to port Open[BGP|OSPF]D)
MFC after: 3 days