[MEDIUM] buffer: make buffer_feed* support writing non-contiguous chunks

[MEDIUM] buffer: make buffer_feed* support writing non-contiguous chunks

The buffer_feed* functions that are used to send data to buffers did only
support sending contiguous chunks while they're relying on memcpy(). This
patch improves on this by making them able to write in two chunks if needed.
Thus, the buffer_almost_full() function has been improved to really consider
the remaining space and not just what can be written at once.