conf: More useful errors for kernels without SO_BINDTODEVICE

Before 5.7, the kernel didn't allow SO_BINDTODEVICE to be called
unprivileged.  That means for earlier kernels, we can't implement binding
listening sockets to a specific interface (e.g. -t %eth0/80).

Currently we'll generate an error on this at the point we actually attempt
the SO_BINDTODEVICE setsockopt(), at which point the connection to the
command line option might not be entirely clear.

Use the fact we now probe for SO_BINDTODEVICE support to make a clearer
error message at the time we parse the forwarding option.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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