Introduce unix socket mode for AFL

[dstepanovsrc]

Add new unix socket mode for AFL to send input not only using stdin or shared file, but using the unix socket communication. The main idea was to use it to fuzz low-level cases like emulated devices or some kind of the network servers. In this case AFL will be independent of the transport level.
We used this approach to fuzz virtio block device implementation for both QEMU and SPDK. Also we presented it on the KVM Forum 2019 event.

Maybe this mode can be helpful for somebody else ).

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[dstepanovsrc]

@googlebot I signed it!

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