Share the server's request-target check as fields::is_request_target()
and use it in write_request_line too. The client previously used
is_field_value(), which let an embedded SP or HTAB through.
encode_path() only escaped CR/LF among the control characters, so with
path encoding enabled a path like "/a\tb" would now be rejected instead
of sent. Percent-encode every control character (0x00-0x1F, 0x7F).