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>

networking/udhcp/d6_dhcpc.c

@@ -1370,7 +1370,13 @@ int udhcpc6_main(int argc UNUSED_PARAM, char **argv)
 					bb_error_msg("no lease, forking to background");
 					client_background();
 					/* do not background again! */
-					opt = ((opt & ~OPT_b) | OPT_f);
+					opt = ((opt & ~(OPT_b|OPT_n)) | OPT_f);
+					/* ^^^ also disables -n (-b takes priority over -n):
+					 * ifup's default udhcpc options are -R -n,
+					 * and users want to be able to add -b
+					 * (in a config file) to make it background
+					 * _and not exit_.
+					 */
 				} else
 #endif
 				if (opt & OPT_n) { /* abort if no lease */

networking/udhcp/dhcpc.c

@@ -1484,7 +1484,13 @@ int udhcpc_main(int argc UNUSED_PARAM, char **argv)
 					bb_error_msg("no lease, forking to background");
 					client_background();
 					/* do not background again! */
-					opt = ((opt & ~OPT_b) | OPT_f);
+					opt = ((opt & ~(OPT_b|OPT_n)) | OPT_f);
+					/* ^^^ also disables -n (-b takes priority over -n):
+					 * ifup's default udhcpc options are -R -n,
+					 * and users want to be able to add -b
+					 * (in a config file) to make it background
+					 * _and not exit_.
+					 */
 				} else
 #endif
 				if (opt & OPT_n) { /* abort if no lease */