copy_file: handle "cp /dev/foo file" (almost) compatibly to coreutils.

(almost because we do not copy mode, which is probably wasn't intended).
+61 bytes.

testsuite/cp/cp-does-not-copy-unreadable-file

@@ -1,6 +1,11 @@
 touch foo
 chmod a-r foo
 set +e
-busybox cp foo bar
+if test `id -u` = 0; then
+    # run as user with nonzero uid
+    setuidgid 1 busybox cp foo bar
+else
+    busybox cp foo bar
+fi
 set -e
 test ! -f bar