nspawn: add --register=yes|no switch to optionally disable registration of the container with machined
util: drop parse_user_at_host() since its unused now
util: modernize readlink_malloc() a bit
machined: optionally, allow registration of pre-existing units (scopes
or services) as machine with machined
efi: fix Undefined reference efi_loader_get_boot_usec when EFI support is disabled
logind: use session_get_state() to get sessions state of the user

In function user_get_state() remove the session_is_active() check, just
count on the session_get_state() function to get the correct session
state.

session_is_active() may return true before starting the session scope
and user service, this means it will return true even before the creation
of the session fifo_fd which will produce incorrect states.

So be consistent and just use session_get_state().
update TODO
machined: fix enumeration of existing machines on restart
units: make use of nspawn's --keep-unit switch in systemd-nspawn@.service
journald: log provenience of signals
logind: always kill session when termination is requested

KillUserProcesses=yes/no should be ignored when termination is
explicitly requested.
logind: ignore PropertiesChanged signals for jobs

Otherwise we get a (harmless) message like:
systemd-logind[30845]: Failed to process message [type=signal sender=:1.36 path=/org/freedesktop/systemd1/job/4674 interface=org.freedesktop.DBus.Properties member=PropertiesChanged signature=sa{sv}as]: Invalid argument
machinectl: add new "machinectl reboot" call
test: fix "make check"

Let's remove the tests for cg_path_get_machine_name(), since they no
longer operate solely on the cgroup path, but actually look up data in
/run. Since we have a test for cg_pid_get_machine_name() this shouldn't
be too much of a loss.