MINOR: hpack: implement the decoder

The decoder is now fully functional. It makes use of the dynamic header
table. Dynamic header table size updates are currently ignored, as our
initially advertised value is the highest we support. Strictly speaking,
the impact is that a client referencing a header field after such an
update wouldn't observe an error instead of the connection being dropped
if it was implemented.

Decoded header fields are copied into a target buffer in HTTP/1 format
using HTTP/1.1 as the version. The Host header field is automatically
appended if a ":authority" header field is present.

All decoded header fields can be displayed if the file is compiled with
DEBUG_HPACK.
diff --git a/include/common/hpack-dec.h b/include/common/hpack-dec.h
new file mode 100644
index 0000000..86bca65
--- /dev/null
+++ b/include/common/hpack-dec.h
@@ -0,0 +1,37 @@
+/*
+ * HPACK decompressor (RFC7541)
+ *
+ * Copyright (C) 2014-2017 Willy Tarreau <willy@haproxy.org>
+ * Copyright (C) 2017 HAProxy Technologies
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef _COMMON_HPACK_DEC_H
+#define _COMMON_HPACK_DEC_H
+
+#include <stdint.h>
+#include <common/config.h>
+#include <common/hpack-tbl.h>
+
+int hpack_decode_frame(struct hpack_dht *dht, const uint8_t *raw, uint32_t len, char *out, int osize);
+
+#endif /* _COMMON_HPACK_DEC_H */