The old epson driver disabled the focus by erasing the setFocus command.
The epson2 driver turned it into a seperate flag. However, scanners
which disabled focus in post_init were not converted to use the new
flag. Because we meanwhile set focus during init, these scanners could
not start anymore. Convert them to use the new flag.

While here, update the comment how we handle focus detection and update
the debug strings accordingly.

Bug reported by Hans Meir and debugged by Zdenek Dohnal. Many thanks!