REORG: include: move common/buffer.h to haproxy/dynbuf{,-t}.h

REORG: include: move common/buffer.h to haproxy/dynbuf{,-t}.h

The pretty confusing "buffer.h" was in fact not the place to look for
the definition of "struct buffer" but the one responsible for dynamic
buffer allocation. As such it defines the struct buffer_wait and the
few functions to allocate a buffer or wait for one.

This patch moves it renaming it to dynbuf.h. The type definition was
moved to its own file since it's included in a number of other structs.

Doing this cleanup revealed that a significant number of files used to
rely on this one to inherit struct buffer through it but didn't need
anything from this file at all.