http2: Improve header parsing

- Error if a header line is larger than supported.

- Warn if cumulative header line length may be larger than supported.

- Allow spaces when parsing the path component.

- Make sure each header line ends in \r\n. This fixes an out of bounds.

- Disallow header continuation lines until we decide what to do.

Ref: https://github.com/curl/curl/issues/659
Ref: https://github.com/curl/curl/pull/663