introduce and use exitcode_t

function                                             old     new   delta
strings_main                                         422     420      -2
setfattr_main                                        175     173      -2
brctl_main                                          1548    1546      -2
makedevs_main                                        979     975      -4
rev_main                                             337     332      -5
getfattr_main                                        307     302      -5
cut_main                                            1201    1196      -5
cksum_main                                           398     393      -5
umount_main                                          573     565      -8
ln_main                                              516     508      -8
expand_main                                          660     652      -8
df_main                                             1068    1060      -8
renice_main                                          346     332     -14
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/13 up/down: 0/-76)            Total: -76 bytes

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