http2: Harden header validation for curl_pushheader_byname

Since we do prefix match using given header by application code
against header name pair in format "NAME:VALUE", and VALUE part can
contain ":", we have to careful about existence of ":" in header
parameter.  ":" should be allowed to match HTTP/2 pseudo-header field,
and other use of ":" in header must be treated as error, and
curl_pushheader_byname should return NULL.  This commit implements
this behaviour.