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

man/man8/Makefile.am

@@ -4,7 +4,41 @@ dist_man_MANS = \
 	vdev_id.8 \
 	zdb.8 \
 	zfs.8 \
+	zfsconcepts.8 \
+	zfsprops.8 \
+	zfs-allow.8 \
+	zfs-bookmark.8 \
+	zfs-change-key.8 \
+	zfs-clone.8 \
+	zfs-create.8 \
+	zfs-destroy.8 \
+	zfs-diff.8 \
+	zfs-get.8 \
+	zfs-groupspace.8 \
+	zfs-hold.8 \
+	zfs-inherit.8 \
+	zfs-list.8 \
+	zfs-load-key.8 \
+	zfs-mount.8 \
 	zfs-program.8 \
+	zfs-project.8 \
+	zfs-projectspace.8 \
+	zfs-promote.8 \
+	zfs-receive.8 \
+	zfs-recv.8 \
+	zfs-redact.8 \
+	zfs-release.8 \
+	zfs-rename.8 \
+	zfs-rollback.8 \
+	zfs-send.8 \
+	zfs-set.8 \
+	zfs-share.8 \
+	zfs-snapshot.8 \
+	zfs-unallow.8 \
+	zfs-unload-key.8 \
+	zfs-unmount.8 \
+	zfs-upgrade.8 \
+	zfs-userspace.8 \
 	zgenhostid.8 \
 	zinject.8 \
 	zpool.8 \