kernel - fix statistics counters for if_bridge.

* Count input statistics in bridge_input() instead of bridge_forward()

* Add output statistics in bridge_forward().  That is, the bridge should
  in abstract look like a piece of hardware from a statistics standpoint,
  so when a packet is forwarded through a bridge interface it needs to
  show up as both an input packet and an output packet.

* Fixes statistics reporting for e.g. 'netstat -in 1'.
kernel: Add three new commands to fctnl

This commit adds the following new commands to fcntl():
  F_DUP2FD         - non-portable functional equivalent of dup2(fd, arg)
  F_DUPFD_CLOEXEC  - A version of F_DUPFD that sets the close-on-exec
                     on the new file descriptor
  F_DUP2FD_CLOEXEC - A version of F_DUP2FD that sets the close-on-exec
                     on the new file descriptor.  It is non-portable

It also adds a missing break in a case statement for F_GETOWN in
sys_fnctl(), spotted by dillon.

reviewed-by: dillon