nixos-generate-config: Don't generate /var/lib/nfs/rpc_pipefs filesystem

It's automatically mounted if the system has support for NFS.
nixos-generate-config: Don't generate filesystem options

We don't want to hardcode configuration options that the current kernel chose
for us when mounting the filesystem, since the defaults can change in the
future.
Merge pull request #1791 from wizeman/u/nixos-gen

nixos-generate-config: improve filesystem generation