mdev: do not follow symlinks in /sys (as was intended prior to rev 18811).

If this breaks things, please document why!
mdev,init: use shared code for fd sanitization

function                                             old     new   delta
bb_daemonize_or_rexec                                155     172     +17
mdev_main                                            500     505      +5
init_main                                            907     856     -51
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 22/-51)            Total: -29 bytes

1 files changed, 8 insertions, 1 deletions

diff --git a/libbb/vfork_daemon_rexec.c b/libbb/vfork_daemon_rexec.c
index 37d4c274e..da0dc03e5 100644
--- a/libbb/vfork_daemon_rexec.c
+++ b/libbb/vfork_daemon_rexec.c
@@ -265,7 +265,14 @@ void FAST_FUNC bb_daemonize_or_rexec(int flags, char **argv)
                 close(2);
         }
 
-        fd = xopen(bb_dev_null, O_RDWR);
+        fd = open(bb_dev_null, O_RDWR);
+        if (fd < 0) {
+                /* NB: we can be called as bb_sanitize_stdio() from init
+                 * or mdev, and there /dev/null may legitimately not (yet) exist!
+                 * Do not use xopen above, but obtain _ANY_ open descriptor,
+                 * even bogus one as below. */
+                fd = xopen("/", O_RDONLY); /* don't believe this can fail */
+        }
 
         while ((unsigned)fd < 2)
                 fd = dup(fd); /* have 0,1,2 open at least to /dev/null */