pciconf: List names of all known extended PCIe capabilities.

Some ids are redundand because the list_ecaps() function decodes them
by explicit switch case. But listing them all makes it easier to not
miss ecaps, while not changing the functionality.

Initial submission by: Dmitry Luhtionov <dmitryluhtionov@gmail.com>
Sponsored by: The FreeBSD Foundation
MFC after: 3 days