Low: nfsserver: fixes issue with proc/fs/nfsd not being mounted during start on some distros

This also addresses a syntax error in the systemd switch case
Low: nfsserver: warn user when nfs port options can not be set

We already document that certain port related options are distro
specific, but attempt to warn a user if we know they've sent an
option that will not work.
Low: nfsserver: fix usage of rquatad_port option
Merge pull request #455 from davidvossel/nfs-cleanup

nfs usage improvements