Revert recent generic::systemuser conversions

This isn't prettier, or more abstract. It's just uglier in syntax,
includes far too much magic that is also wrong. It's also conceptually
wrong as it adds a membership to a virtual supplementary group of
"systemusers" that won't and can't include all system users.

The right way to go here is to keep using system => true to mark system
users, that would in turn use adduser & adduser.conf's FIST_SYSTEM_UID &
LAST_SYSTEM_UID to designate whether a user is system or not.

This reverts the conversions for the following users:
icinga, mirror, gitblit, ipython, librenms, limn, ocg, tcpircbot.

This reverts commits:
  - 13932dcfe3e092dc23c63f0df643c0408d46f351
  - dcf55c490c5c2990c807e3d692bba872981b8952
  - b66eb8c7a54d0e55899149d04435eec448d11865
  - 76fdd4a29d892089c8da9f41b0c2247abfa1fd35
  - b6f8462bb68a248b20b031ffcbc227688e707804
  - 98e6ab557e7b113e1724572910ce0a9cb7eaf14d
  - 9c5fc3843246f894a5919164ae10c429c983db4f
  - 3ba59bc5dc341e1a5e51c6a62c8cfbcb4146d53b
  - 3df220dbe688896723b3975131a7427d8c9860c7
  - 80d3f1fcb9be8a4087652b5200c8dc8c66a1db1b

Change-Id: I8445bf3a8fb1a47385aa36d226fe5291c2cacc1e