Without this change, arbitrary MAC addresses will always be masked.

This change restores the intended behaviour.

*This condition will probably never be met, but it is theoretically
possible that the code could still fail at this point. Catch all errors,
and exit, ruthlessly.

Another oversight in my error handling.

The code was only checking whether all of the bytes were read,
but there are other errors that can be caught via errno.

Enforce strict errno handling, when generating random
numbers for command `setmac`.

previously, it was always initialised, but now it's only
initialised if '?' is used on a mac address character in
command `setmac`

this is done by simply moving mac address character
randomisation to a separate function