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

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

WARNS=3'ify.

Repo copy libpthreads to libkse.
This introduces the WITHOUT_LIBKSE nob,
and changes WITHOUT_LIBPTHREADS to mean with neither threading libs.
Approved by: re(kensmith)

Add preliminary sparc64 support to libpthread. This does not
yet work, but hopefully someone familiar with the sparc64
port can pick up the reins.

Submitted by: jake
With mods by: deischen