ash,kill: use common code for kill applet/builtin

ash,kill: use common code for kill applet/builtin

# make bloatcheck
function                                             old     new   delta
evaltreenr                                           644     654     +10
evaltree                                             644     654     +10
parse_conf                                          1440    1444      +4
dpkg_deb_main                                        426     429      +3
ed_main                                             3319    3321      +2
passwd_main                                         2093    2091      -2
kill_main                                            830     826      -4
singlemount                                         4609    4601      -8
find_command                                         962     954      -8
get_lcm                                              123     105     -18
.rodata                                           132243  132147     -96
killcmd                                              449     120    -329
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/7 up/down: 29/-465)          Total: -436 bytes

# size busybox_old busybox_unstripped
   text    data     bss     dec     hex filename
 723901    2940   27504  754345   b82a9 busybox_old
 723457    2940   27504  753901   b80ed busybox_unstripped