MINOR: xprt: add a new show_fd() helper to complete some "show fd" dumps.

MINOR: xprt: add a new show_fd() helper to complete some "show fd" dumps.

Just like we did for the muxes, now the transport layers will have the
ability to provide helpers to report more detailed information about their
internal context. When the helper is not known, the pointer continues to
be dumped as-is if it's not NULL. This way a transport with no context nor
dump function will not add a useless "xprt_ctx=(nil)" but the pointer will
be emitted if valid or if a helper is defined.