WTS: Fix crashers and infinite loop regexp and minor tweaks.

* en:Skaak and a few other pages were crashing in RT-testing
  Added a couple missing DU.isElt(node) checks.

* en:Grizz and Dot Com and other pages were stuck during
  serializing because of a known regexp issue that sends
  the RE engine into an infinite loop.  Fixed the couple of
  regexp uses that had not been fixed.

* In emitSeparator, check for presence of page source before
  attempting to use original source separator -- this is required
  to prevent crashing of html2wt of a html snippet passed into
  'node parse' on the command line.

* Added FIXMES about missing node argument to callbacks in a
  couple places so that original separator text is used there.
  (FIXMEs worth addressing).

* No change in parser test results.

Change-Id: I8d41ee3cf8a98308baea2e5d95cf5f2b24423dc8