Added functions to read/write the PCI config space.
Fixed memory-account bug for mmapphys().
Be careful that the device exists when we start the irq-thread.
Removed print in tcpip.cc. We already have that in link.cc.
pci: support multiple devices with same class/subclass.
Fixed overflow-bug in cp.
tcp: use shared memory when reading from NIC device.
tcp: fixed handling of dropped packets.
tcp: improved printing of circular buffer.
ne2k: ensure to handle every flag in ISR.

It might change during the interrupt handling and thus we have
to read it again until there is no flag set anymore.
ne2k: ensure that the IRQ is registered if the first arrives.
Moved common functionality of lo and ne2k into lib.
ne2k: ensure that interrupts are enabled in PCI config space.
Added e1000 driver.