MINOR: mux-quic/h3: send SETTINGS as soon as transport is ready

MINOR: mux-quic/h3: send SETTINGS as soon as transport is ready

As specified by HTTP3 RFC, SETTINGS frame should be sent as soon as
possible. Before this patch, this was only done on the first qc_send()
invocation. This delay significantly SETTINGS emission until the first
H3 response is ready to be transferred.

This patch fixes this by ensuring SETTINGS is emitted when MUX-QUIC is
being setup.

As a side point, return value of finalize operation is checked. This
means that an error during SETTINGS emission will cause the connection
init to fail.

This should be backported up to 2.7.