Fix the bug that stream is closed with wrong error code

priority_spec::valid(): remove const qualifier from return value

This commit fixes the bug that stream is closed with wrong error code
(0).  This happens when STREAM or DATA frame with END_STREAM flag set
is received and it violates HTTP messaging rule (i.e., content-length
does not match) and the other side of stream has been closed.  In this
case, nghttp2_on_stream_close_callback should be called with nonzero
error code, but previously it is called with 0 (NO_ERROR).

gcc generates warning:
* type qualifiers ignored on function return type [-Wignored-qualifiers]

Faster Huffman encoding/decoding

Avoid filename collision of static and dynamic lib

Add new flag ENABLE_STATIC_CRT for Windows

Renames the output of the ENABLE_STATIC_LIB library/archive output
to nghttp2_static.lib/.a to avoid filenames colliding with the output
name for ENABLE_SHARED_LIB library/archive, when both are enabled.
Signed-off-by: William A Rowe Jr <wrowe@pivotal.io>
Signed-off-by: Yechiel Kalmenson <ykalmenson@pivotal.io>

Previously, mruby context is wrongly shared by multiple patterns if
the underlying SharedDownstreamAddr is shared by multiple
DownstreamAddrGroups.  This commit fixes it.

This change adds the CMake option;

  ENABLE_STATIC_CRT  Build libnghttp2 against the MS LIBCMT[d]

This avoids linking to msvcrt.lib for binaries to compile (/MT[d])
and link against the static C Runtime libcrt.lib, and
avoiding the msvcrt[d].dll dependency.
Signed-off-by: William A Rowe Jr <wrowe@pivotal.io>
Signed-off-by: Yechiel Kalmenson <ykalmenson@pivotal.io>

This is the second attempt.  The first attempt was
8a59ce6d and it failed.

This reverts commit 8a59ce6d.

Reported in https://github.com/nghttp2/nghttp2/issues/1384

Fix request stall if backend connection is reused and buffer is full.

Support building nghttpx with systemd using cmake

Run nghttpx integration test with cmake build.  Autotools build erases
artifacts for packaging tests, so we cannot run nghttpx integration
test with autotools build at the moment.

Relay on go modules to download dependencies.