Makefile: cleanup usage of libdir and install user units

  Actually, /usr/lib/systemd is an API-like path, there is no variables
in /lib/ part - hardcode it.
  On the other hand, PAM's libdir could be lib or lib64.
Use pkg-config to determine that.
  Lastly, install system-wide user units, too.
Merge pull request #32 from zdzichu/master

Makefile: cleanup usage of libdir and install user units