The existing Ethernet connection implementation is PCAP-based. This
allows bridging your virtual machine directly to the host's network, but
requires elevated permissions on your OS and appropriate drivers which
often aren't available on Wi-Fi cards.
This commit adds a libslirp-based backend that provides just enough
internal networking to handle TCP/IP out of the guest to host, LAN or
Internet. As such things like ICMP, IPX and NetBIOS don't work properly.
The use case for this connection is to give Internet access to the guest
for things like IRC, HTTP and other connections to online services.
Currently the ne2000 emulator explicitly depends on pcap.
Decouple the ne2000 code and pcap code so we can always build with the
emulator but optionally disable pcap.
This also updates build defines and configuration to make this
decoupling explicit to the user.
