Fix the new NFS client so that it handles the "nfs_args" value
in mnt_optnew. This is needed so that the old mount(2) syscall
works and that is needed so that amd(8) works. The code was
basically just cribbed from sys/nfsclient/nfs_vfsops.c with minor
changes. This patch is mainly to fix the new NFS client so that
amd(8) works with it. Thanks go to Craig Rodrigues for helping with
this.

Tested by: Craig Rodrigues (for amd)
MFC after: 2 weeks