Add support for options to 'sharesmb'.
Fixes: #1182
+ Move nfs.c:foreach_nfs_shareopt() to libshare.c:foreach_shareopt()
  so that it can be (re)used in smb.c (and later iscsi.c).
+ Call net(8) with the guest_ok option.
+ Rewrite the sharesmb part of the zfs(8) manpage.
  + Add more examples
  + Inform about the (new) options to sharesmb.
+ Add STDERR_VERBOSE to libzfs_run_process() so that we
  can catch any errors from net(8).
+ Don't call smb_retrieve_shares() after enabling a share.
  No point, it's done when really needed anyway.
+ Support 'y', 'yes' and 'true' for guest_ok.
* If sharesmb=on, just exit cleanly directly at the top of
  get_smb_shareopts_cb().
* Remove param 2 and 3 from smb_enable_share_one(). That is
  availible in the first param - the impl_share struct.
* Extra debugging added in smb_enable_share_one() - print
  all our options just before we call net(8).