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

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
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 22/-51)            Total: -29 bytes