user defined custom groups and normal system groups can have same name

[jnweiger]

Seen with customgroups 0.8.0 (but probably not a regression)

• enable customgroups
• admin creates a user bob, and adds him to bob-group.
• user bob creates a customgroup 1234 and adds user admin. OK
• user bob creates a customgroup bob-group and adds user admin. BAD.
• user admin creates group 1234 and and adds user bob. BAD.

Listing users and groups and custom groups is now very confusing.

Expected behaviour

• it should not be possible to create a customgroup when a system group with the same name already exists.
• it should not be possible to create a system group, when a customgroup with the same name already exists.