Since 7d84439 ("BUILD: hpack: include global.h for the trash that is needed
in debug mode"), hpack decode tool fails to compile on targets that enable
USE_THREAD. (ie: linux-glibc target as reported by Christian Ruppert)

When building hpack devtool, we are including src/hpack-dec.c as a dependency.
src/hpack-dec.c relies on the global trash whe debug mode is enabled.
But as we're building hpack tool with a limited scope of haproxy
sources, global trash (which is declared in src/chunk.c) is not available.
Thus, src/hpack-dec.c relies on a local 'trash' variable declared within
dev/hpack/decode.c

This used to work fine until 7d84439.
But now that global.h is explicitely included in src/hpack-dec.c,
trash variable definition from decode.c conflicts with the one from global.h:

  In file included from include/../src/hpack-dec.c:35,
                   from dev/hpack/decode.c:87:
  include/haproxy/global.h:52:35: error: thread-local declaration of 'trash' follows non-thread-local declaration
     52 | extern THREAD_LOCAL struct buffer trash;

Adding THREAD_LOCAL attribute to 'decode.c' local trash variable definition
makes the compiler happy again.

This should fix GH issue #2009 and should be backported to 2.7.

The few hpack development tools are now integrated into the main
makefile, which allows to remove the original one which was causing
lots of build warnings. A README was added to explain how to build
instead.

The following directories were moved from contrib/ to dev/ to make their
use case a bit clearer. In short, only developers are expected to ever
go there. The makefile was updated to build and clean from these ones.

base64/  flags/  hpack/  plug_qdisc/  poll/  tcploop/  trace/