Remove the legacy "utmp" API, now that our utils have been moved to utmpx.
As I wrote in UPGRADING, the main change for "normal" users is probably
that /var/log/wtmp is no longer logged to, just /var/log/wtmpx.
Leave the old API in libc as compat symbols that do nothing and always
succeed for the run-time linker (i.e. old binaries still using them).
dports has been kindly adjusted by zrj and new binary packages for master
are on their way.
A future commit will provide a wtmpcvt(1) utility to convert old wtmp
files to the wtmpx format.