Returns nonzero if the *value* which is supposed to be the value of the
:authority or host header field is valid according to
https://tools.ietf.org/html/rfc3986#section-3.2

Note that :authority and host field values are not authority.  They
do not include userinfo in RFC 3986, see
https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.2, that
is, it does not include '@'.  This function treats '@' as a valid
character.

*value* is valid if it merely consists of the allowed characters.
In particular, it does not check whether *value* follows the syntax
of authority.