comm clientinfo: Provide clientinfo interface

This will allow a comm client to provide information from its
configuration dicts to the comm, so that a comm can be shared by
multiple clients but still have client-specific configuration.

Currently, this new interface has no callers, and no consumers.
No comm clients call comm_if->_clientinfo(); they all just pass NULL.
No comms take any notice of the clientinfo information; they always
return NULL for the clientinfo*.

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