Detached address check

["Robert Swindells" <rjs%fdy2.co.uk@localhost>]

I feel that the changes made to check whether a sending address is valid
are too strict.

The check means that an attempt to connect from the address of an
interface that is marked detached to the same address will fail even
though there is still a route for this address to the loopback device.

The change was introduced in:

<http://mail-index.netbsd.org/source-changes/2016/09/15/msg077726.html>

Robert Swindells