fifo_buffer: add functions _capacity() and _available()

src/fifo_buffer.h

@@ -62,6 +62,19 @@ fifo_buffer_new(size_t size);
 void
 fifo_buffer_free(struct fifo_buffer *buffer);
 
+/**
+ * Return the capacity of the buffer, i.e. the size that was passed to
+ * fifo_buffer_new().
+ */
+size_t
+fifo_buffer_capacity(const struct fifo_buffer *buffer);
+
+/**
+ * Return the number of bytes currently stored in the buffer.
+ */
+size_t
+fifo_buffer_available(const struct fifo_buffer *buffer);
+
 /**
  * Clears all data currently in this #fifo_buffer object.  This does
  * not overwrite the actuall buffer; it just resets the internal