Update ZFS manual pages to a mdoc(7) reimplementation.

Update ZFS manual pages to a mdoc(7) reimplementation.

The zfs(8) and zpool(8) manual pages now match the state of the ZFS module
and have been customized for FreeBSD.

The new texts of the "Deduplication" subsection in zfs(8), the zpool "split"
command, the zfs "dedup" property and several other missing parts have been
added from illumos or OpenSolaris snv_134 (CDDL-licensed).

The mdoc(7) reimplementation of whole manual pages, the descriptions of the
zpool "readonly" property, "zfs diff" command and descriptions of several
other missing command flags and/or options were authored by myself.

MFC after:	1 week