Reorganise printing

Printer drivers don't keep the device open all the time any more; it is
opened on each use.  This is to support improved error reporting:
"device missing", "out of paper", and so on.

The improved error reporting isn't implemented yet.
Check printer status

When using /dev/lp? or /dev/usblp?, check the printer status (out of
paper, error light on, etc.) before attempting to print.