BUG/MINOR: h3: reject request URI with invalid characters

BUG/MINOR: h3: reject request URI with invalid characters

Ensure that the HTX start-line generated after parsing an HTTP/3 request
does not contain any invalid character, i.e. control or whitespace
characters.

Note that for now path is used directly as URI. Thus, the check is
performed directly over it. A patch will change this to generate an
absolute-form URI in most cases, but it won't be backported to avoid
configuration breaking in stable versions.

This must be backported up to 2.6.