- rewrite run-parts

- rewrite run-parts
   text    data     bss     dec     hex filename
   1029       0       0    1029     405 debianutils/run_parts.o-old
    478       0       0     478     1de debianutils/run_parts.o-new-bare
    600       0       0     600     258 debianutils/run_parts.o-new-full

bare, i.e. without long opts and fancy stuff
./scripts/bloat-o-meter bb_old busybox_unstripped   function                                             old     new   delta
act                                                    -     215    +215
run_parts_main                                       216     201     -15
valid_name                                            50       -     -50
runparts_long_options                                 64       -     -64
.rodata                                           124323  124163    -160
run_parts                                            513       -    -513
------------------------------------------------------------------------------
(add/remove: 1/3 grow/shrink: 0/2 up/down: 215/-802)         Total: -587 bytes