We removed the concept of facts, so we need to update the existing
io.systemd.Basic facts provider to metrics. This commit does just
that. Its mostly mechanical.
This also means that facts.{c,h} and varlink-io.systemd.Facts.{c,h}
are gone now.
The collection of facts is entirely unprivileged and has very little to
do with PID1. PID1 is privileged and single-threaded and a point of
contention, so we shouldn't put things in PID1 that don't need to be
there. A separate service can be enabled/disabled/started/stopped at
will, is easy to sandbox, etc. If it turns out to be necessary to
collect some facts through PID1 in the future, we can always add a
smaller facts endpoint to PID1 again.
