more precise detection of weave-attached-ness
Rather than looking for the host end of the veth pair, we look for the
expected interface inside the container. This has the advantage of
detecting weave-attached-ness even when the container is sharing a
network namespace with others.

Closes #303, #302.
Support detaching on containers created in container networking mode.
Merge pull request #304 from viirya/fix_detach

Support detaching on containers created in container networking mode.
correctly check whether weavedns is running
and return an error status rather than exit when containers stop just
before an HTTP call, which in turn allows us to make tell_dns properly
ignore weavedns container absences.

Fixes #300