CHANNELv4: expose the underlying flow via an accessor
basic_ethif test: make the dependencies portable
Also add an HTTP callback to the basic stack
Merge branch 'split-libs' of github.com:avsm/mirage into split-libs
Add a basic-http test
better build constraints
Merge branch 'split-libs' of https://github.com/avsm/mirage into split-libs

Conflicts:
	lib_test/basic_stackv4/config.ml
Complete the port to the new network stack.

We now just miss the combinators for HTTP
Remove spaces
Port the tests to the new combinators

still need to port the HTTP test though