Make netgroup support optional
On at least Linux/musl and Linux/uclibc, netgroup support is not available. PolKit fails to compile on these systems for that reason.
This change makes netgroup support conditional on the presence of the setnetgrent(3) function which is required for the support to work. If that function is not available on the system, an error will be returned to the administrator if unix-netgroup: is specified in configuration.
Fixes bug 50145.
Closes #14.
Signed-off-by: A. Wilcox