homed: make it easier to run multiple instances of homed

When debugging homed while being logged into a user account manged by
homed it is a good idea to be able to run a second copy of homed. In
order to not collide with its AF_UNIX socket and bus name use, let's add
a new env var $SYSTEMD_HOME_DEBUG_SUFFIX, when set the busnames/socket
names are suffixed by it. When setting this while debugging one can
invoke an additional copy without interfering with the host one.