Provide a new name `polypath' in the global dictionary which can be
made to produce new closures of type `comm'. These comms send packets
via multiple local interfaces.
Currently this module is not very useful because we lack a suitable
`monitor-command' for enumerating local interfaces.
We disable it if we don't have CONFIG_IPV6 because then we don't have
adns_text2addr.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>