extend my_gethwaddr on linux to see disabled interfaces

currently my_gethwaddr on linux uses SIOCGIFHWADDR to list all interfaces. That only shows interfaces that are UP.

For my_gethwaddr it is absolutely unnecessary limitation. It needs any mac address of any interface. This limitation causes it to fail on systems that have all interfaces disabled for some reason (or only virtual interfaces are UP - in certain virtual machines).

It'd be better to change my_gethwaddr to examine all interfaces, UP or DOWN.

A relatively easy way of doing it would be to read /sys/class/net/*/address files.

Once this is fixed, it should be checked if the following three suppressions
can be removed from @antipatterns in mysql-test-run.pl:

     qr|feedback plugin: failed to retrieve the MAC address|,

     qr|Plugin 'FEEDBACK' init function returned error|,

     qr|Plugin 'FEEDBACK' registration as a INFORMATION SCHEMA failed|,