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

include/libzfs.h

@@ -815,6 +815,13 @@ void libzfs_free_str_array(char **strs, int count);
 
 int libzfs_envvar_is_set(char *envvar);
 
+/*
+ * Utility functions for zfs version
+ */
+extern void zfs_version_userland(char *, int);
+extern int zfs_version_kernel(char *, int);
+extern int zfs_version_print(void);
+
 /*
  * Given a device or file, determine if it is part of a pool.
  */