MINOR: quic: rename frontend sock-per-conn setting

MINOR: quic: rename frontend sock-per-conn setting

On frontend side, a quic_conn can have a dedicated FD or use the
listener one. These different modes can be activated via a global QUIC
tune setting.

This patch adjusts the option. First, it is renamed to the more
meaningful name 'tune.quic.fe.sock-per-conn'. Also, arguments are now
either 'default-on' or 'force-off'. The objective is to better highlight
reliationship with 'quic-socket' bind option.

The older option is deprecated and will be removed in 3.5.