text-based mount command: Function to stuff "struct pmap" from mount options

Both the text-based mount.nfs command and the umount.nfs command need
to fill in a pmap structure based on string mount options.  Introduce
a shared function that can do this.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>