gallium: rename pipe_buffer_handle to pipe_buffer, rework pipebuffer/ code

gallium:  rename pipe_buffer_handle to pipe_buffer, rework pipebuffer/ code

Provide an actual definition of the pipe_buffer struct,  containing
the parameters used to create the buffer, and its refcount.

Shift refcounting buffers out of the winsys interface, similar to
surfaces & textures.

Rework pipebuffer/ to reflect the fact these changes, and also Michel's
reworking of the buffer interface.