Complete the CloudABI networking refactoring.

Now that all of the packaged software has been adjusted to either use
Flower (https://github.com/NuxiNL/flower) for making incoming/outgoing
network connections or can have connections injected, there is no longer
need to keep accept() around. It is now a lot easier to write networked
services that are address family independent, dual-stack, testable, etc.

Remove all of the bits related to accept(), but also to
getsockopt(SO_ACCEPTCONN).