Support custom systemd unit directory (e.g. it's /usr/lib/systemd on openSUSE, then `make SYSTEMD_UNIT_DIR=/usr/lib/systemd install` should be executed.

* Fixed typo: did not unblock shutdown when systemd unit was in failed state
* Allow custom systemd unit directory in Makefile

disable swapoff by default (it's not a good idea to try to turn swap off in a random moment of time)