top: implement scrolling up/down (_very_ useful)

top: implement scrolling up/down (_very_ useful)

function                                             old     new   delta
handle_input                                         494     564     +70
top_main                                             928     947     +19
display_topmem_process_list                          363     381     +18
display_process_list                                1442    1453     +11
clearmems                                             38      28     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/1 up/down: 118/-10)           Total: 108 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>