Update stable/10 from 10.3-STABLE to 10.4-PRERELEASE as part of
the 10.4 release cycle, also belatedly marking the official start
of the code slush.

Set the default mdoc(7) version to 10.4, and update the clang(1)
TARGET_TRIPLE to reflect 10.4. While at it, add missing FreeBSD
major versions to mdoc(7).

Approved by: re (implicit)