find: implement -executable

find: implement -executable

function                                             old     new   delta
func_executable                                        -      19     +19
parse_params                                        1505    1519     +14
static.params                                        216     228     +12
packed_usage                                       32847   32855      +8
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/0 up/down: 53/0)               Total: 53 bytes

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