MINOR: applet: Add API functions to manipulate input and output buffers

MINOR: applet: Add API functions to manipulate input and output buffers

In this patch, some functions were added to ease input and output buffers
manipulation, regardless the corresponding applet is using its own buffers
or it is relying on channels buffers. Following functions were added:

  * applet_get_inbuf  : Get the buffer containing data pushed to the applet
                        by the stream

  * applet_get_outbuf : Get the buffer containing data pushed by the applet
                        to the stream

  * applet_input_data : Return the amount of data in the input buffer

  * applet_skip_input : Skips <len> bytes from the input buffer

  * applet_reset_input: Skips all bytes from the input buffer

  * applet_output_room: Returns the amout of space available at the output
                        buffer

  * applet_need_room  : Indicates that the applet have more data to deliver
                        and it needs more room in the output buffer to do
			so