pciconf: extend tree mode to also print numerical IDs

Also print the vendor/device and subvendor/subdevice IDs in addition
to any strings from the database found if the -v flag is given more
than once.
This helps with device identification if the strings resolve to
identical values for entire product families as well as when the
exact card cannot be determined from the string.
In theory a second call to pciconf could present that information in
non-tree mode but that kind-of defeats the purpose.

Reviewed by: jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D56248