This API function with nonzero |val| parameter disables HTTP Messaging
validation in nghttp2 library, so that application can use nghttp2
library for non-HTTP use.

Previously we did not check HTTP semantics and it is left out for
application.  Although checking is relatively easy, but they are
scattered and error prone.  We have implemented these checks in our
applications and also feel they are tedious.  To make application
development a bit easier, this commit adds basic HTTP semantics
validation to library code.  We do following checks:

server:

* HEADERS is either request header or trailer header.  Other type of
header is disallowed.

client:

* HEADERS is either zero or more non-final response header or final
  response header or trailer header.  Other type of header is
  disallowed.

For both:

* Check mandatory pseudo header fields.
* Make sure that content-length matches the amount of DATA we
  received.

If validation fails, RST_STREAM of type PROTOCOL_ERROR is issued.

This reverts commit d45f5a51.

Previously if HTTP/1 proxy is used for backend connection, we read all
incoming bytes from proxy including response body, which may be part
of HTTP/2 protocol.  While investigating this issue, we found that
http_parser_execute() returns 1-less length when we call
http_parser_pause() inside on_headers_complete callback.  To
workaround this, we increment the return value by 1.  This commit also
fixes possible segmentation fault error, which could be caused by the
lack of stopping libev watcher in disconnect().