Align mount options handling and type/function names with OpenZFS

Refactor the temporary mount option in a way which minimizes
differences with upstream.  Additionally, replace the zfs_sb_t
type with zfsvfs_t and rename several functions to be consistent
with the upstream names.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5876