text-based mount options: Use new pmap stuffer when rewriting mount options

all nfs_options2pmap() in nfs_rewrite_mount_options() instead of
open-coding the logic to convert mount options to a pmap struct.
The new nfs_options2pmap() function is more careful about avoiding
invalid mount option values, and handles multiply-specified transport
protocol options correctly.

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