core: do not assert when sysconf(_SC_NGROUPS_MAX) fails (#4466)

core: do not assert when sysconf(_SC_NGROUPS_MAX) fails (#4466)

Remove the assert and check the return code of sysconf(_SC_NGROUPS_MAX).

_SC_NGROUPS_MAX maps to NGROUPS_MAX which is defined in <limits.h> to
65536 these days. The value is a sysctl read-only
/proc/sys/kernel/ngroups_max and the kernel assumes that it is always
positive otherwise things may break. Follow this and support only
positive values for all other case return either -errno or -EOPNOTSUPP.

Now if there are systems that want to re-write NGROUPS_MAX then they
should not pass SupplementaryGroups= in units even if it is empty, in
this case nothing fails and we just ignore supplementary groups. However
if SupplementaryGroups= is passed even if it is empty we have to assume
that there will be groups manipulation from our side or the kernel and
since the kernel always assumes that NGROUPS_MAX is positive, then
follow that and support only positive values.