find: add optional support for '-exec ... {} +'

function                                             old     new   delta
do_exec                                                -     309    +309
parse_params                                        1416    1487     +71
find_main                                            342     406     +64
packed_usage                                       29958   30014     +56
func_exec                                            138     127     -11
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(add/remove: 1/0 grow/shrink: 3/1 up/down: 500/-11)           Total: 489 bytes

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>