relay_read_http: tighten up header parsing

relay_read_http: tighten up header parsing

1) reject headers with embedded NULs
2) reject headers with invalid characters in the name
3) reject Transfer-Encoding with values other than "chunked"
4) reject chunk values containing non-hex characters
5) reject Content-Length values of "+0" or "-0"
6) reject requests without a ' ' and headers without a ':'

Reported by Ben Kallus, OK bluhm@