group: Fix nonposix, posix and external handling and conversions

The nonposix, posix and external parameters need to be mutually
exclusive. external was missing in this list. Only one of the three
parameters can be used.

external can not be set to no/false. This results in an error now.

if nonposix is used, posix is set as not nonposix. The nonposix
parameter is not used within the code anymore..

New tests have been added to tests the addition of users with for
nonposix and posix groups. The tests for the external group is not
active due to the need of an AD.

Fixes: #528 (Error creating nonposix group)