via nmount(), build up an iovec where each iovec member is an NFS mount
option, and pass the iovec down to the kernel via nmount(). These options
are then parsed in the kernel.
This should make it easier to add new NFS mount options in future.
Many, many thanks to Doug Rabson for taking my initial patches,
and cleaning them up. In addition, Doug added a fallback_mount()
function so that the newer mount_nfs program will work against older
kernels, to facilitate upgrading/downgrading scenarios.
Doug also re-wrote the mount_nfs.8 man page.
Reviewed by: dfr
4b5bc28358