Gerber: reset aperture numbers on initialization.

This prevents aperture numbers from growing to infinity if
you export several times to Gerber in the same GUI session.

Acked-by: Andrew Poelstra <apoelstra@wpsoftware.net>
Acked-by: Peter Stuge <peter@stuge.se>
pcb-printf: Add 0.1mil resolution, improve documentation and a small cleanup.

0.1mil resolution is needed for improved imperial drill file generation.
Apart form this, the list of %m unit modifiers is now fully
documented and a redundant test was removed.

Acked-by: Andrew Poelstra <apoelstra@wpsoftware.net>
Acked-by: Peter Stuge <peter@stuge.se>
Gerber: use 0.1mil resolution for imperial drill files.

Acked-by: Andrew Poelstra <apoelstra@wpsoftware.net>
Acked-by: Peter Stuge <peter@stuge.se>