polypath: Provide privsep mode

If secnet is going to drop privileges, it won't be able to call
setsockopt(,,SO_BINDTODEVICE,) to set up the new polypath sockets.

Provide an arrangement where this is done by a child forked before we
drop privilege.

Add some comments to the existing just-broken-down interface change
handling code, saying whether and how they are used in the non-privsep
and privsep cases.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>