brctl: make "show" command retrieve data from /sys

brctl: make "show" command retrieve data from /sys

ioctl interface is obsolete and has no 32/64 compat shim,
making "brctl show" fail for 32-bit userspace and 64-bit kernel.

function                                             old     new   delta
show_bridge                                            -     310    +310
read_file                                              -      64     +64
if_indextoname                                       117       -    -117
brctl_main                                          1183     885    -298
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/1 up/down: 374/-415)          Total: -41 bytes

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