Translate the errno to gctl_error() texts.

Spotted by:	mwlucas
Report the 1-based key numbers rather than the 0-based ones to be consistent.

Fix documentation for destroy command.  Not sure how the wrong explanation
happened.

Spotted by:	mwlucas
Remove unused 'struct route' fields.
Remove unused 'struct route *' argument from nd6_output_flush().