systemctl: add a way to explicitly request client-side unit installing

systemctl: add a way to explicitly request client-side unit installing

This adds support for a new environment variable
SYSTEMCTL_INSTALL_CLIENT_SIDE, that ensures that systemctl executes
install operations client-side instead of passing them to PID1. This is
useful in debugging situations, but even beyond that. However, we don't
want to make it official API, hence let's just make it an undocumented
environment variable.

Similar, add a second variable, SYSTEMCTL_SKIP_SYSV which allows
skipping the SysV chkconfig fall-back if set. This is useful for similar
reasons, and exposed as undocumented as environment variable for similar
reasons, too.