Add Unix socket forwarding

[blaubaer]

Reimplements gliderlabs/ssh#196 by @deansheather for the current fork architecture.

Summary

• add explicit callbacks and handlers for OpenSSH direct and reverse Unix socket forwarding
• integrate reverse Unix forwards with connection workers, channel limits, and the shared reverse-forward limit
• handle the x/crypto/ssh listener-registration race and prevent stale channel opens from crossing path reuse
• add a standalone streamlocal example plus protocol, lifecycle, limit, race, and real Unix-socket tests

Security

• keep Unix forwarding disabled unless callbacks and handlers are explicitly configured
• leave path authorization, listener creation, permissions, stale files, and unlink behavior to application policy
• avoid exposing callback/backend errors to SSH clients

Testing

• go test -count=1 ./...
• go test -shuffle=on -count=10 ./...
• go test -race -count=1 ./...
• targeted streamlocal tests with 100 normal and 20 race repetitions
• go vet and golangci-lint
• Linux 386, Windows amd64/386, and Darwin amd64/arm64 cross-builds
• all standalone example modules