systemd: add logind configuration options

Make the following logind.conf options configurable in NixOS:

  HandleHibernateKey=
  HandleLidSwitch=
  HandlePowerKey=
  HandleSuspendKey=

All options use the same default values as upstream[1], so this commit
should not make logind behave any different than before (unless the
options are changed).

[1]: http://www.freedesktop.org/software/systemd/man/logind.conf.html