udhcpc: don't exit after backgrounding on -n -b

udhcpc: don't exit after backgrounding on -n -b

Currently, running "udhcpc -n -b" causes udhcpc to go to background and
then exit after some time unless a lease is obtained.

It's not very useful to do so
as the calling process doesn't know
if the lease was obtained or not anyway.

The code actually tries to favor "-b" over "-n",
but doesn't clear "-n" flag while clearing "-b" after backgrounding.

So, clear "-n" flag after going into background.
This effectively makes "-b" override "-n" completely
and "-n -b" behave the same as "-b".

This allows to override default "-n" option, passed to udhcpc by ifupdown,
without recompiling busybox.

URL: https://bugs.busybox.net/11691

Signed-off-by: Andrey Mazo <ahippo@yandex.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>