Fix following Info cross-references to anchors

Fix following Info cross-references to anchors

* lisp/info.el (Info-read-subfile): Add to the returned value the
length of subfile preamble, after converting it to file's byte
offset, as expected by the caller.  Use bufferpos-to-filepos.
(Info-find-node-2): If searching for a node with a
1000-character slop fails, try again with a 10000-character slop,
to account for known bugs in Texinfo 5.0 and 5.1.  (Bug#21055)

* lisp/international/mule-util.el (bufferpos-to-filepos): New
function.
* etc/NEWS: Mention bufferpos-to-filepos.