ifupdown: kill -TERM is default, so drop -TERM

ifupdown: kill -TERM is default, so drop -TERM
ifupdown: ferror() does not set errno, don't use perror
ifupdown: make netmask parsing smaller and more strict
(was accepting 255.0.255.0, 255.1234.0.0 etc...)

function                                             old     new   delta
count_bits                                            36       -     -36
execute                                             2573    2535     -38
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-74)             Total: -74 bytes
   text    data     bss     dec     hex filename
 684204    2744   14000  700948   ab214 busybox_old
 684132    2744   14000  700876   ab1cc busybox_unstripped