The command `echo -- -g` prints `-- -g` so the generated `groupadd` command
was syntactically incorrect and failing. Solution was to remove `--` since
echo command does not understand it, and add instead an extra space before
`-g` to avoid echo interpreting it as a flag.
- add new --state parameter and support for deleting groups
- add new --system parameter to create system groups if supported by the
os
- remove special case for legacy redhat/centos versions, they are long
gone
Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>