Fix .ent and .dtd file path to work on R15B and later

The .dtd and .ent files used to generate the FAQ (and Erlang
documentation) were moved somewhere between R14B03 and R15B03.

If xsltproc can't find them, it silently generates broken output.
One symptom of breakage is that names like Björn get turned into
Bjrn.

This patch adds a check to make sure the .dtd and .ent files are
there.
Explain that Erlang now has more unicode support
Update the information about embedded systems

- Pocket PCs are passe, don't bother talking about them
- Embedded systems are less exotic now, name some
Update the current version of Erlang; it's now R16B
Update links to the ITESM Erlang course
Fix broken links

Some have moved, some aren't relevant any more
Remove defunct companies from the "who uses Erlang" list

Schlund was acquired a few years ago; unclear if they still use Erlang

Textendo have a placeholder webpage and mail to
Jeroen Koops (my contact there) bounces.
Fix broken links

Some links could just be moved. Others were deleted:

- Everything on erlang-projects.org is deleted; the site has
  been down for a long time.
- The 'TCM' tool is well and truly dead