toXML: `\n` not escaped, `'` wrongly escaped

[yorickvP]

My grub config file was evaluated differently in fix. It turns out fix is missing \n -> 
 escaping in xml in writeXmlEscaped and has extra ' -> ' escaping.

Repro:

builtins.toXML {
  a_newline = "x\ny";
  b_apostrophe = "x'y";
  c_tab = "x\ty";
  d_cr = "x\ry";
  e_others = "x&<>\"y";
}