Proper fix for "unknown argument" error message

Proper fix for "unknown argument" error message

While commit 1b8210f848631 kind of fixed the corner-case of invalid
short-options packed with others, it broke error reporting for
long-options. Revert it and deploy a proper solution:

When passing an invalid short-option, e.g. 'iptables -vaL', getopt_long
sets the variable 'optopt' to the invalid character's value. Use it for
reporting instead of optind if set.

To distinguish between invalid options and missing option arguments,
ebtables-translate optstring needs adjustment.

Fixes: 1b8210f848631 ("ebtables: Fix error message for invalid parameters")
Signed-off-by: Phil Sutter <phil@nwl.cc>