Markup substitution through HelpWriterContext.

Markup substitution through HelpWriterContext.

Make all calls to process the in-source strings to help output go
through HelpWriterContext.  This makes it easy to implement common
functionality using C++ mechanisms.

Moved all the HTML link processing stuff into HelpWriterContext as well
and cleaned up links.dat.  The generated HTML pages should no longer
contain broken links (but links between program pages are not there
yet).

The markup substitution itself is still done by functions in wman.cpp;
will move those into private functions in helpwritercontext.cpp as a
separate change.

Related to #685 and #969.

Change-Id: I37d45549214e49a6edab82fb64ec7b38b9e72094