include: Move QemuLockCnt APIs to their own header

Currently the QemuLockCnt data structure and associated functions are
in the include/qemu/thread.h header.  Move them to their own
qemu/lockcnt.h.  The main reason for doing this is that it means we
can autogenerate the documentation comments into the docs/devel
documentation.

The copyright/author in the new header is drawn from lockcnt.c,
since the header changes were added in the same commit as
lockcnt.c; since neither thread.h nor lockcnt.c state an explicit
license, the standard default of GPL-2-or-later applies.

We include the new header (and the .c file, which was accidentally
omitted previously) in the "RCU" part of MAINTAINERS, since that
is where the lockcnt.rst documentation is categorized.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20240816132212.3602106-7-peter.maydell@linaro.org

util/async.c

@@ -30,6 +30,7 @@
 #include "block/graph-lock.h"
 #include "qemu/main-loop.h"
 #include "qemu/atomic.h"
+#include "qemu/lockcnt.h"
 #include "qemu/rcu_queue.h"
 #include "block/raw-aio.h"
 #include "qemu/coroutine_int.h"