Restructure mount option handling

Restructure mount option handling

Restructure the handling of mount options to be consistent with
upstream OpenZFS.  This required making the following changes.

- The zfs_mntopts_t was renamed vfs_t and adjusted to provide
  the minimal needed functionality.  This includes a pointer
  back to the associated zfsvfs_t.  Plus it made it possible
  to revert zfs_register_callbacks() and zfsvfs_create() back
  to their original prototypes.

- A zfs_mnt_t structure was added for the sole purpose of
  providing a structure to pass the osname and raw mount
  pointer to zfs_domount() without having to copy them.

- Mount option parsing was moved down from the zpl_* wrapper
  functions in to the zfs_* functions.  This allowed for the
  code to be simplied and it's where similar functionality
  appears on other platforms.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>