Add ZFS usage tips to freebsd-tips.

Add a bunch of examples on how to use ZFS features like:

• listing available space,
• setting and displaying a userquota,
• displaying pool I/O statistics and pool history,
• displaying the compression ratio for a dataset,
• various list options (sorting, removing headers),
• performing a dry-run of a snapshot delete,
• removing a range of snapshots,
• setting a custom property,
• preventing removal of a snapshot with ZFS holds,
• permission sets for zfs send/receive.

Additionally, clarify the existing examples a bit when
it comes to displaying space by mentioning UFS explicitly.
Other examples include displaying I/O in top(1), querying
sysctl(8) for active CPUs and available RAM. Mention systat(1)
and its options, too.
While here, reformat the example to upload a dmesg(8) a bit
to wrap properly.

Thanks to Allan Jude for his help with some of the ZFS examples.

Reviewed by: dru,allanjude
Approved by: allanjude (earlier version)
Relnotes: yes (ZFS examples in freebsd-tips)
Differential Revision: https://reviews.freebsd.org/D18541