udevd: cleanup std{in,our,err} on startup

udevd: cleanup std{in,our,err} on startup

It occurs, when root-partition has no /dev/console, meaning that kernel
could not open it, and such udevd is started without open filedescriptors
0 1 2. In that case udevd openes its sockets (netlink and control). They
get fds between 0 and 2. Later duping /dev/null to 0 1 2 closes the sockets
and replaces them with /dev/null.

The error condition can also be reproduced by starting udevd with this
command-line:
  udevd --daemon <&- >&- 2>&-