Expand the set of APIs available for locating PCI capabilities:

• pci_find_extcap() is repurposed to be used for fetching PCI-express extended capabilities (PCIZ_* constants in <dev/pci/pcireg.h>).
• pci_find_htcap() can be used to locate a specific HyperTransport capability (PCIM_HTCAP_* constants in <dev/pci/pcireg.h>).
• Cache the starting location of the PCI-express capability for PCI-express devices in PCI device ivars.