Since last commit there's no point anymore in having two variants of the
same function, let's switch to b_free() only. __b_drop() was renamed to
__b_free() for obvious consistency reasons.
A small race exists in buffers with "show sess all". This one wants to show
some information grabbed from the buffer (especially in HTX mode). But the
thread owning this buffer might just be releasing its area, right after a
free() or munmap() call, resulting in a head that is not seen as empty yet
though the area was released. It may then be dereferenced by "show sess all"
causing a crash. Note that in practice it only happens in debug mode with
UAF enabled, but it's tricky enough to fix it right now.
This should be backported to stable versions which support threads and a
store barrier. It's worth noting that by performing the clearing first,
b_free() and b_drop() now become two exact equivalent.
This function will be used to move parts of a buffer to another place
in the same buffer, even if the parts overlap. In order to keep things
under reasonable control, it only uses a length and absolute offsets
for the source and destination, and doesn't consider head nor data.
