MINOR: quic: convert startup check in a freestanding function

MINOR: quic: convert startup check in a freestanding function

quic_test_socketopts() function is used to detect system support for
QUIC network stack. Previously, it relies on an already bound listener
instance, notably to ensure that two UDP sockets can be bound on the
same source address.

Improve quic_test_socketopts() to run without any listener argument. It
now automatically instantiates and manipulates two dummy sockets FDs to
check for multi-bind support. This brings two advantages :
* the function is now called via an initcall
* it will easily be reusable to implement build option description