let ga_init() fail gracefully if getgrouplist does

let ga_init() fail gracefully if getgrouplist does

Apparently getgrouplist() can fail on OSX for when passed a non-existent
group name. Other platforms seem to return a group list consisting of
the numeric gid passed to the function.

This makes ga_init() handle this failure case gracefully, where it will
return success but with an empty group list array.

bz3848; ok dtucker@