MINOR: server: reject QUIC servers without explicit SSL

MINOR: server: reject QUIC servers without explicit SSL

Report an error during server configuration if QUIC is used by SSL is
not activiated via 'ssl' keyword. This is done in _srv_parse_finalize(),
which is both used by static and dynamic servers.

Note that contrary to listeners, an error is reported instead of a
warning, and SSL is not automatically activated if missing. This is
mainly due to the complex server configuration : _srv_parse_finalize()
is ideal to affect every servers, including dynamic entries. However, it
is executed after server SSL context allocation performed via
<prepare_srv> XPRT operation. A proper fix would be to move SSL ctx
alloc in _srv_parse_finalize(), but this may have unknown impact. Thus,
for now a simpler solution has been chosen.