#2884: Improve configure_voms error reporting

There are at least two things we could do to improve the usability of
configure_voms.

First, we should investigate whether it's possible to move the GetOptions() call
and the usage check to before all others statements that cause the script to
exit. Sometimes, users call "configure_voms --help" but it never gets there.

Second, whenever we write to the post-install log, it would be helpful to write
a short summary and/or pointer to stdout. There are some places in the script
where this happens already, so just do more of that.