MINOR: connection: add a few functions to report the data and xprt layers' names

MINOR: connection: add a few functions to report the data and xprt layers' names

These functions will be needed by "show sess" on the CLI, let's make them
globally available. It's important to note that due to the fact that we
still do not set the data and transport layers' names in the structures,
we still have to rely on some exports just to match the pointers. This is
ugly but is preferable to adding many includes since the short-term goal
is to get rid of these tests by having proper names in place.