busybox

ifupdown: rewrite state file atomically

By user's request.
Decided to not use fcntl(F_SETLKW) in lieu of problems with locking
on networked filesystems. The existence of /var/run/ifstate.new
is treated as a write lock. rename() provides atomicity.

function                                             old     new   delta
ifupdown_main                                       1019    1122    +103

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