nfs-common: Convert /etc/default/nfs-* on upgrade

Ben Hutchingsrequested to merge
benh/convert-conf into master

Since nfs-utils 1.3.5-rc2, the upstream systemd unit files ignore the environment variables that they used to append to daemon command lines. We used to define those variables in /run/sysconfig/nfs based on the contents of /etc/default/nfs-{common,kernel-server}.

If these files have been changed from the default, run a script on upgrade that converts the command-line options to a configuration file /etc/nfs.conf.d/local.conf.

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