services: nfs: Expose configuration options for TCP and UDP.

* gnu/services/nfs.scm (<nfs-configuration>)[nfsd-tcp?, nfsd-udp?]: New
fields.
(nfs-shepherd-services)[rpc.nfsd]: Populate the --{no-,}tcp and --{no-,}udp
command line options based on the values of the nfsd-tcp? and nfsd-upd?
fields.
* doc/guix.texi (Network File System): Add doc.