Returns nonzero if the *value* which is supposed to be the value of
the :path header field is valid according to
https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2.3

*value* is valid if it merely consists of the allowed characters.
In particular, it does not check whether *value* follows the syntax
of path.  The allowed characters are all characters valid by
`nghttp2_check_header_value` minus SPC and HT.