lib-http: Restructured the header, message, request, and response parsers to have an extensible and consistent API using flags. Extended the test suites with a few cases that test parsing with and without the STRICT flag.

lib-http: Added option to the header parser to make it lenient towards illegal characters in header field content. The offending characters are then skipped without error. This is required for the http client as a workaround for bugs in certain HTTP servers. This behavior is explicitly not enabled for the http-request-parser as used by our own HTTP server implementation.

lib-http: Implemented limits on overall HTTP header size, size of individual header fields and the number of fields in the header.

lib-http: Added initial HTTP client implementation.