Implement drmParsePciBusInfo and drmParsePciDeviceInfo.

Uses drmGetBusid for bus info, and pciN with libpci for device info.

Aims to address PR pkg/51795 without needing the kernel changes
suggested in PR kern/51786.  Compile-tested only sofar -- should
confirm before closing any PRs.

(Sorry it took so long for me to find a round tuit!)

XXX pullup-7
XXX pullup-8

...along with src/external/mit/xorg/lib/libdrm/Makefile r1.16 for
libpci.

(riastradh)