Rewrite of nfs.c to keep options per host separated.

Example: Setting sharenfs to

  "rw=@192.168.69.8,no_subtree_check,no_root_squash,ro=@192.168.69.45,no_subtree_check,root_squash"

will create the option string

  sec=sys,ro,no_subtree_check,no_root_squash,mountpoint,no_subtree_check,no_root_squash,no_subtree_check,root_squash

which is then run on _each host_!. Part of this is "default" options.

Instead, go through the option string, look for hosts and then make
sure options for each host is kept apart by using a linked list
(maintained by the ZFS list_* functions). Also, the default options
should only be added to the list of options if setting sharenfs to
"on".

Closes: #2773