main: make busybox.conf mode handling less obscure

main: make busybox.conf mode handling less obscure

function                                             old     new   delta
static.mode_mask                                       -      20     +20
main                                                 782     785      +3
static.mode_chars                                     15      13      -2
run_applet_no_and_exit                               450     441      -9
mode_mask                                             24       -     -24
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 1/2 up/down: 41/-53)            Total: -12 bytes

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