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>