core: rework crash handling

core: rework crash handling

This introduces a new systemd.crash_reboot=1 kernel command line option
that triggers a reboot after crashing.

This also cleans up crash VT handling. Specifically, it cleans up the
configuration setting, to be between 1..63 or a boolean. This is to
replace the previous logic where "-1" meant disabled. We continue to
accept that setting, but only document the boolean syntax instead.

This also brings the documentation of the default settings in sync with
what actually happens.

The CrashChVT= configuration file setting is renamed to CrashChangeVT=,
following our usual logic of not abbreviating unnecessarily. The old
setting stays support for compat reasons.

Fixes #1300