(Review) (FACT-683) Use XPG4 variant of `id` in Solaris to determine GID

Prior to this commit, the GID fact would attempt to invoke the `/usr/bin/id`
variant of the `id` command in Solaris, which does not include the necessary flags to
determine the user GID in Solaris 10.

This commit updates the fact to instead use the more advanced `/usr/xpg4/bin/id`
for Solaris instead.
The `/usr/xpg4/id` comes from `SUNWxcu4` package which is an addon package distributed in the solaris installation media. Notably it is not present in the slim install. So we need to be careful with that. I think that requiring `SUNWxcu4` as a prerequisite for puppet shouldn't be a problem. For solaris 11, we need to check if xpg4 is distributed in the base install.