Adjust chunked encoding handling.

Adjust chunked encoding handling.

Add an extra state to distinguish in between chunks CRLF handling from
the last chunk which can optionally have some trailer fields included.
rpki-client ignores these trailer header fields (they are also not common
it seems).
Also remove the empty line handling in http_parse_chunked() for explicit
checks in http_read(). Because of the extra state the switch back to
non-chunked mode can now be delayed until the transfer is over.

OK tb@