tcp, udp: Bind outbound listening sockets by interface instead of address

Currently, outbound forwards (-T, -U) are handled by sockets bound to the
loopback address.  Typically we create two sockets, one for 127.0.0.1 and
one for ::1.

This has some disadvantages:
 * The guest can't connect via 127.0.0.0/8 addresses other than 127.0.0.1
 * We can't use dual-stack sockets, we have to have separate sockets for
   IPv4 and IPv6.

The restriction exists for a reason though.  If the guest has any
interfaces other than pasta (e.g. a VPN tunnel) external hosts could reach
the host via the forwards.  Especially combined with -T auto / -U auto this
would make it very easy to make a mistake with nasty security implications.

We can achieve this a different way, however.  Don't bind to a specific
address, but _do_ use SO_BINDTODEVICE to restrict the sockets to the "lo"
interface.  We fall back to the old behaviour for older kernels where
SO_BINDTODEVICE is not available unprivileged.

Note that although traffic to a local but non-loopback address is passed
over the 'lo' interface (as seen by netfilter and dumpcap), it doesn't
count as attached to that interface for the purposes of SO_BINDTODEVICE
(information from the routing table overrides the "physical" interface).
So, this change doesn't help for bug 100.

It's also not a complete fix for bug 113, it does however:
 * Get us a step closer to fixing bug 113
 * Slightly simplify the code
 * Make things a bit easier to allow more flexible binding on the guest in
   in future

Link: https://bugs.passt.top/show_bug.cgi?id=113

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

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