libbb: xfunc_error_retval can be a byte

libbb: xfunc_error_retval can be a byte

function                                             old     new   delta
xfunc_die                                             55      60      +5
log_and_exit                                          43      45      +2
hush_main                                            961     963      +2
xfunc_error_retval                                     4       1      -3
tty_main                                              87      84      -3
sv_main                                             1183    1180      -3
sort_main                                            804     801      -3
run_applet_no_and_exit                               440     437      -3
puts_maybe_newline                                   198     195      -3
nohup_main                                           228     225      -3
httpd_main                                           753     750      -3
get_header_tar                                      1496    1493      -3
fsck_minix_main                                     2972    2969      -3
expr_main                                            110     107      -3
cmp_main                                             641     638      -3
chat_main                                           1359    1356      -3
builtin_exit                                          48      45      -3
arping_main                                         1587    1584      -3
BB_EXECVP_or_die                                      47      44      -3
send_cgi_and_exit                                    901     895      -6
diff_main                                           1197    1191      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/18 up/down: 9/-60)            Total: -51 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>