Reorganize zfs(8) man page into sections

Reorganize zfs(8) man page into sections

Most subcommands got their own manpages (e.g. create). Some related
commands grouped into a single manpage and symlinks created (e.g. set,
get, and inherit). I did this when topics were either too short to
warrant their own file or so interrelated that a user would want to
refer between commands in the same file.

Corrected .Sx internal references to .Xr cross refs; lots of .Sx
references from when text was all in zfs.8 needed to be changed to
.Xr zfs-$SUBCOMMAND 8 cross references.

Divided subcommand list in zfs(8) into sections of related
functionality. This required writing new descriptions for some
commands.

Preserved ".Os Linux", `.Os` macro parsing behavior differs between
mandoc from the "BSD" mandoc package (available on Ubuntu) and man
from Ubuntu's man-db package, which calls groff to format the manpages.

Groff handles the `.Os` macro differently and wrongly, defaulting
it to "BSD" in `/usr/share/groff/*/tmac/mdoc/doc-common`, instead of
getting the default from `uname`.

A future set of changes will introduce build-time preprocessing of
manpages for platform-specific documentation and can insert the
correct operating system name.

Added SEE ALSO sections, the newly-divided zfs-*.8 subcommand man
pages needed their own SEE ALSO sections pointing to related
subcommands and, in some cases, documentation from other packages
(e.g. zfs-share.8).

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ross Williams <ross@ross-williams.net>
Closes #9559