PCI bridge support

[richardg867]

This adds:

• Support for multiple PCI bus numbers in pci.c
• Emulation of the DEC/Intel 21150 PCI bridge (9 bus masters, so 9 children)
• Automatic insertion of a 21150 PCI bridge upon reaching the last NORMAL slot (never run out of slots)
• Proper emulation of AGP bridges on Intel and VIA chipsets
• A few IRQ steering fixes on AGP machines

Notes:

• The meaning of the pci_add_card return value has changed to accomodate for multiple bus numbers. The data type and usage haven't changed.
• Having a PCI device on the AGP bridge is legal at least on Intel platforms. See the Intel C440GX+ motherboard which uses a 21150 bridge attached to the AGP bus providing 66 MHz PCI slots.
• On the other hand, VIA AGP bridges accept PCI devices but not PCI bridges ("not enough resources..."), so the PCI slot on VIA AGP bridges is given a special flag to prevent the automatic 21150 from reaching it. Some VIA machines also don't have IRQ steering table entries for the AGP slot (???)
• Some devices may not like sitting behind bridges. ES1371 gameport is one of them. Can be fixed with a pci.c overhaul, as there is no clean way of rearranging PCI cards right now.
• If the amount of PCI devices matches the amount of PCI slots, the last device will be alone behind a 21150 bridge. Not a big deal but can be fixed with a pci.c overhaul.
• Additionally, having just enough PCI devices may result in an empty 21150 bridge being added on VIA AGP systems, due to the no-bridges quirk on the VIA AGP bridges. Same as above, not a big deal but can be fixed with a pci.c overhaul.
• The PCI_NO_BRIDGES machine PCI type flag disables the automatic 21150 bridge system. It's present just in case there's some BIOS out there that can't handle bridges properly.