Propagate $RPCGSSDOPTS from /etc/default to systemd service script. (See: #846950)

Hi,

Pursuant to Salvatore's request this fixes, in a minimal way, a naming inconsistency that keeps command line arguments for rpc.gssd from being propagated between

    /etc/default/nfs-common

and

    /lib/systemd/system/rpc-gssd.service

Users of separate keytabs in /etc/keytabs still have to make other manual changes, but this fixes the error that is hardest to track down. It is a typo, and has remained unresolved for over for years!

Whenever nfs-utils is upgraded, I fix it manually on all my systems. As someone who filed or amended the involved bugs years ago ago, I disagreed with the administrative downgrade by Gaudenz Steinlin. Raising them again was on my todo list for bullseye.

Please allow at least this small correction of a typo into the upcoming release. Thank you!

Eventually, it would be better to integrate Joachim Falk's merge request into the changes are pending from upstream. I did not yet test it, but he seems to address the failing conditions that cause systemd to quit due to some paths not existing in my use case.