Fix process-specific values in nickle-generated footprints

Fix process-specific values in nickle-generated footprints

This centralizes the definition of the process values in the
footprint.5c file, instead of scattering the values across each
footprint program. This fixes several footprints with insufficient
clearance or mask width while shrinking excessive values in other
footprints.

This also adds some convenience functions to footprint.5c for
generating ground pads, both large square ones with vias embedded and
smaller ones that just want less paste on the pad than a normal pad of
similar size.

Signed-off-by: Keith Packard <keithp@keithp.com>