MINOR: applet: Add API functions to get data from the input buffer

MINOR: applet: Add API functions to get data from the input buffer

There was already functions to pushed data from the applet to the stream by
inserting them in the right buffer, depending the applet was using or not
the legacy API. Here, functions to retreive data pushed to the applet by the
stream were added:

  * applet_getchar   : Gets one character

  * applet_getblk    : Copies a full block of data

  * applet_getword   : Copies one text block representing a word using a
                       custom separator as delimiter

  * applet_getline   : Copies one text line

  * applet_getblk_nc : Get one or two blocks of data

  * applet_getword_nc: Gets one or two blocks of text representing a word
                       using a custom separator as delimiter

  * applet_getline_nc: Gets one or two blocks of text representing a line