This change fixes the bug that stream is out of dependency tree if the
number of nodes in a dependency tree which we add new node to is
already maximum (NGHTTP2_MAX_DEP_TREE_LENGTH) and the number of
maximum concurrent streams is more than more than
NGHTTP2_MAX_DEP_TREE_LENGTH.

* Add NGHTTP2_HTTP_1_1_REQUIRED error code
* Allow transmission of WINDOW_UPDATE on reserved (remote)
* Allow reception of WINDOW_UPDATE on reserved (local)
* Treat frame larger than MAX_FRAME_SIZE as FRAME_SIZE_ERROR

ALPN identifier is still h2-14 to continue interop, since draft-14 and
-15 are binary compatible.  The new error code was added in draft-15,
but HTTP/2 allows extensions can freely add new error code, so it is
not a problem.

This bug was found by scan-build

Merge branch 'header_add_override' of https://github.com/kennypeng/nghttp2 into kennypeng-header_add_override