cmd: unify unix-specific code

cmd/publish_unix.go

@@ -0,0 +1,10 @@
+// +build darwin linux dragonfly freebsd netbsd openbsd
+
+package cmd
+
+import "syscall"
+
+func processExists(pid int) bool {
+	err := syscall.Kill(pid, syscall.Signal(0))
+	return err == nil
+}