Change nfsserver so that it uses the nfssvc() system call provided
in sys/nfs/nfs_nfssvc.c by registering with it using the
nfsd_call_nfsserver function pointer. Also, add the build glue for
nfs_nfssvc.c optionally based on "nfsserver" and also as a loadable
module.

Submitted by: rmacklem
Reviewed by: kib
Approved by: kib (mentor)