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.