use %m printf specifier where appropriate

use %m printf specifier where appropriate

function                                             old     new   delta
out                                                   85      75     -10
udhcpd_main                                         1472    1461     -11
open_stdio_to_tty                                     98      85     -13
init_exec                                            245     232     -13
udhcpc_main                                         2763    2749     -14
do_cmd                                              4771    4755     -16
status_line_bold_errno                                32      14     -18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/7 up/down: 0/-95)             Total: -95 bytes

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