- Update stable/10 to 10.1-PRERELEASE now that the
  code slush is in effect.  (Forgotten on the 22nd.)
- Set the 10.1 as the .Fx mdoc(7) default.
- Update the TARGET_TRIPLE and BUILD_TRIPLE for clang(1)
  to reflect 10.1.

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