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

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

Replace utxrm(8) by utx(8).

At first, I added a utility called utxrm(8) to remove stale entries from
the user accounting database. It seems there are cases in which we need
to perform different operations on the database as well. Simply rename
utxrm(8) to utx(8) and place the old code under the "rm" command.

In addition to "rm", this tool supports "boot" and "shutdown", which are
going to be used by an rc-script which I am going to commit separately.

Remove wtmpcvt(1).

The wtmpcvt(1) utility converts wtmp files to the new format used by
utmpx(3). Now that HEAD has been branched to stable/9 and 9.0 is
released, there is no need for it in HEAD.

MFC after: never

Add WITHOUT_UTMPX switch to the build system.

This knob removes the tools that are exclusively used to view and
maintain the databases maintained by utmpx, namely last, users, who,
wtmpcvt, ac, lastlogin and utxrm.

The tool w is not in this list, because it has some other functionality
which is unrelated to utmpx; it is hardlinked to the uptime tool.