MINOR: buffers: Move swap_buffer into buffer.c and add deinit_buffer function

swap_buffer is a global variable only used by buffer_slow_realign. So it has
been moved from global.h to buffer.c and it is allocated by init_buffer
function. deinit_buffer function has been added to release it. It is also used
to destroy the buffers' pool.
diff --git a/include/common/buffer.h b/include/common/buffer.h
index f8bd26e..f1edae0 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -53,6 +53,7 @@
 extern struct list buffer_wq;
 
 int init_buffer();
+void deinit_buffer();
 int buffer_replace2(struct buffer *b, char *pos, char *end, const char *str, int len);
 int buffer_insert_line2(struct buffer *b, char *pos, const char *str, int len);
 void buffer_dump(FILE *o, struct buffer *b, int from, int to);