Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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 utmpx stub from libulog.

I'm not increasing the shlib major version for this, because not a
single application outside the base system should have used these
functions in such a short timespan.

Rewrite ulog_login(3) and ulog_logout(3) to build on top of the utmpx
implementation in libc.

Add a libutempter compatibility interface to libulog.

The ulog_login_pseudo(3) and ulog_logout_pseudo(3) interfaces provide a
functionality identical to what libutempter has to offer. Just transform
libutempter's calls into the before mentioned functions.

libutempter doesn't work with utmpx, so instead of fixing I thought the
easiest way would be to integrate this functionality. libutempter is
used by applications like xterm and the KDE libraries, so if I ever
change the underlying file format, these applications will keep working
automatically.

Also increase __FreeBSD_version to indicate the addition (as well as the
import of libulog).