MINOR: quic: startup detect for quic-conn owned socket support

MINOR: quic: startup detect for quic-conn owned socket support

To be able to use individual sockets for QUIC connections, we rely on
the OS network stack which must support UDP sockets binding on the same
local address.

Add a detection code for this feature executed on startup. When the
first QUIC listener socket is binded, a test socket is created and
binded on the same address. If the bind call fails, we consider that
it's impossible to use individual socket for QUIC connections.

A new global option GTUNE_QUIC_SOCK_PER_CONN is defined. If startup
detect fails, this value is resetted from global options. For the
moment, there is no code to activate the option : this will be in a
follow-up patch with the introduction of a new configuration option.

This change is part of quic-conn owned socket implementation.
It may be backported to 2.7 after a period of observation.