Add option [-V|--version] to emit version string

Add the 'zfs version' and 'zpool version' subcommands to display
the version of the user space utilities and loaded zfs kernel
module.  For example:

$ zfs version
zfs-0.8.0-rc3_169_g67e0366b88
zfs-kmod-0.8.0-rc3_169_g67e0366b88

The '-V' and '--version' aliases were added to support the
common convention of using 'zfs --version` to obtain the version
information.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: TerraTech <1118433+TerraTech@users.noreply.github.com>
Closes #2501
Closes #8567

man/man8/zfs.8

@@ -38,7 +38,7 @@
 .Nd configures ZFS file systems
 .Sh SYNOPSIS
 .Nm
-.Fl ?
+.Fl ?V
 .Nm
 .Cm create
 .Op Fl p
@@ -327,6 +327,8 @@
 .Fl i
 .Op Fl l
 .Ar filesystem
+.Nm
+.Cm version
 .Sh DESCRIPTION
 The
 .Nm
@@ -2453,6 +2455,13 @@ original form.
 Displays a help message.
 .It Xo
 .Nm
+.Fl V, -version
+.Xc
+An alias for the
+.Nm zfs Cm version
+subcommand.
+.It Xo
+.Nm
 .Cm create
 .Op Fl p
 .Oo Fl o Ar property Ns = Ns Ar value Oc Ns ...
@@ -4624,6 +4633,13 @@ Indicates that zfs should make
 inherit the key of its parent. Note that this command can only be run on an
 encryption root that has an encrypted parent.
 .El
+.It Xo
+.Nm
+.Cm version
+.Xc
+Displays the software version of the
+.Nm
+userland utility and the zfs kernel module.
 .El
 .Sh EXIT STATUS
 The