Rewrote merge_files function to handle case when multiple modules claim

Rewrote merge_files function to handle case when multiple modules claim
they support a partical PCI device.  For example, the 8139cp and 8139too
modules both claim they can handle the generic RTL 8139 eth chipset.

Also incorporate support to distinguish when a driver can only handle
specific subvendor/subdevice IDs.  E.g., the bcm5700 driver currently
does not handle all broadcom devices---some must still be handled by
tg3 (e.g., on the Dell PowerEdge 850).