CLEANUP: channel: use "channel" instead of "buffer" in function names

This is a massive rename of most functions which should make use of the
word "channel" instead of the word "buffer" in their names.

In concerns the following ones (new names) :

unsigned long long channel_forward(struct channel *buf, unsigned long long bytes);
static inline void channel_init(struct channel *buf)
static inline int channel_input_closed(struct channel *buf)
static inline int channel_output_closed(struct channel *buf)
static inline void channel_check_timeouts(struct channel *b)
static inline void channel_erase(struct channel *buf)
static inline void channel_shutr_now(struct channel *buf)
static inline void channel_shutw_now(struct channel *buf)
static inline void channel_abort(struct channel *buf)
static inline void channel_stop_hijacker(struct channel *buf)
static inline void channel_auto_connect(struct channel *buf)
static inline void channel_dont_connect(struct channel *buf)
static inline void channel_auto_close(struct channel *buf)
static inline void channel_dont_close(struct channel *buf)
static inline void channel_auto_read(struct channel *buf)
static inline void channel_dont_read(struct channel *buf)
unsigned long long channel_forward(struct channel *buf, unsigned long long bytes)

Some functions provided by channel.[ch] have kept their "buffer" name because
they are really designed to act on the buffer according to some information
gathered from the channel. They have been moved together to the same place in
the file for better readability but they were not changed at all.

The "buffer" memory pool was also renamed "channel".
diff --git a/include/types/channel.h b/include/types/channel.h
index 2ef3b3e..c68f4f7 100644
--- a/include/types/channel.h
+++ b/include/types/channel.h
@@ -137,7 +137,7 @@
  * Those bits indicate that there are some processing to do on the buffer
  * contents. It will probably evolve into a linked list later. Those
  * analysers could be compared to higher level processors.
- * The field is blanked by buffer_init() and only by analysers themselves
+ * The field is blanked by channel_init() and only by analysers themselves
  * afterwards.
  */
 #define AN_REQ_DECODE_PROXY     0x00000001  /* take the proxied address from a 'PROXY' line */