helper/proc: move package out of internal

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/proc/files.go

@@ -0,0 +1,17 @@
+package proc
+
+import (
+	"os"
+	"os/exec"
+)
+
+func ExtraFile(cmd *exec.Cmd, f *os.File) (fd uintptr) {
+	return ExtraFileSlice(&cmd.ExtraFiles, f)
+}
+
+func ExtraFileSlice(extraFiles *[]*os.File, f *os.File) (fd uintptr) {
+	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
+	fd = uintptr(3 + len(*extraFiles))
+	*extraFiles = append(*extraFiles, f)
+	return
+}