BUG/MEDIUM: h3: reject request with invalid header name

BUG/MEDIUM: h3: reject request with invalid header name

Reject request containing invalid header name. This concerns every
header containing uppercase letter or a non HTTP token such as a space.

For the moment, this kind of errors triggers a connection close. In the
future, it should be handled only with a stream reset. To reduce
backport surface, this will be implemented in another commit.

Thanks to Yuki Mogi from FFRI Security, Inc. for having reported this.

This must be backported up to 2.6.