add settings flags, refactor lenient_http_headers setting

[jasnell]

A previous change during a security fix added the ability to set a lenient_http_headers flag on the HTTP Parser instance. The way it was done was a bit of a hack at the time in order to avoid an ABI breaking change. This PR adds a new uint32_t flags field to http_parser_settings in order to provide a more robust and correct way of passing these kinds of settings into a parser instance.

Note: This is an ABI change so it would require a semver-major bump

/cc @indutny