MFC 299205: Restore name=value format of PCI location strings.

When devctl was added, the location string for PCI devices was changed to
use the PCI "selector" that pciconf and devctl accept. However, devd
assumes that location strings are formatted as a list of name=value pairs.
As a result, devd is no longer parsing any of the values out of PCI
device events. Restore the previous format of the PCI location strings
to restore the location and slot keywords in case any devd scripts are
using this. Add the "selector" as a new 'dbsf' location variable.