brctl: convert remaining commands to work via /sys

brctl: convert remaining commands to work via /sys

function                                             old     new   delta
write_ulong                                            -      96     +96
show_bridge                                          310     338     +28
static.ops                                             3       -      -3
arm_ioctl                                             20       -     -20
packed_usage                                       33344   33315     -29
brctl_main                                           885     721    -164
------------------------------------------------------------------------------
(add/remove: 1/2 grow/shrink: 1/2 up/down: 124/-216)          Total: -92 bytes
   text	   data	    bss	    dec	    hex	filename
 982112	    485	   7296	 989893	  f1ac5	busybox_old
 982157	    485	   7296	 989938	  f1af2	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>