cmd: unify unix-specific code

cmd/publish_linux.go

@@ -1,8 +0,0 @@
-package cmd
-
-import "syscall"
-
-func processExists(pid int) bool {
-	err := syscall.Kill(pid, syscall.Signal(0))
-	return err == nil
-}

cmd/publish_darwin.go → cmd/publish_unix.go

@@ -1,3 +1,5 @@
+// +build darwin linux dragonfly freebsd netbsd openbsd
+
 package cmd
 
 import "syscall"

cmd/subscribe_linux.go → cmd/subscribe_unix.go

@@ -1,3 +1,5 @@
+// +build linux dragonfly freebsd netbsd openbsd
+
 package cmd
 
 const (