MINOR: connection: add conn_fd() to retrieve the FD only when it exists

MINOR: connection: add conn_fd() to retrieve the FD only when it exists

There are plenty of places (particularly in debug code) where we try to
dump the connection's FD only when the connection is defined. That's
already a pain but now it gets one step further with QUIC because we do
*not* want to dump this FD in this case.

conn_fd() checks if the connection exists, is ready and is not fd-less,
and returns the FD only in this case, otherwise returns -1. This aims at
simplifying most of these conditions.