proc: cleaner extra files

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/proc/files.go

@@ -0,0 +1,13 @@
+package proc
+
+import (
+	"os"
+	"os/exec"
+)
+
+func ExtraFile(cmd *exec.Cmd, f *os.File) (fd uintptr) {
+	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
+	fd = uintptr(3 + len(cmd.ExtraFiles))
+	cmd.ExtraFiles = append(cmd.ExtraFiles, f)
+	return
+}