homed: allow overriding the root directory for home dirs via env var (i.e. use a...

homed: allow overriding the root directory for home dirs via env var (i.e. use a different path than /home/)

This is a debugging feature. It's sometimes incredibly useful to be able
to run a second instance of homed that operates on another dir than
/home/.

Specifically, if you build homed from the source tree you can now run an
instance of it pretty reasonably directly from the build tree via:

  sudo SYSTEMD_HOME_DEBUG_SUFFIX=foo SYSTEMD_HOMEWORK_PATH=$(pwd)/build/systemd-homework SYSTEMD_HOME_ROOT=/home/foo ./build/systemd-homed

And then talk to it via

  sudo SYSTEMD_HOME_DEBUG_SUFFIX=foo homectl …

(you might need to tweak your dbus policy for this to work fully though)