Make mdrun print the list of compatible GPUs

This extends functionality so that the available, compatible, and
actually-used GPU IDs are reported when that makes sense. This will
help (e.g.) gmx tune-pme optimize resource usage by running mdrun
to probe for the available hardware.

The old implementation and documentation had a few issues that have
been corrected or updated. The major issue was that the list of
compatible device IDs was formed and then replaced by the list of
device IDs to use, so the former was no longer available during
subsequent output.

Change-Id: I3ca3e88054bd7d288f556af7e5d7704145ab6fca