nixos nfs: allow setting the lockd ports.

This helps in setting a fixed firewall open port for NFS lockd.

Based on:
http://rlworkman.net/howtos/NFS_Firewall_HOWTO

(cherry picked from commit b32ca0616ff70795f71995fa79ea508b82f30b3a)

Conflicts:
	nixos/modules/services/network-filesystems/nfsd.nix