[OpenSER-Docs] module documentation sgml to xml migration

[Daniel-Constantin Mierla]

Hello,

many thanks to Edson for scripts and Henning for help and testing, the 
first phase of migrating the sgml to xml is done. You can now add 
changes to any module documentation you wish.

Remaining steps, that should not affect the content:
- migration from entities to xinclude (Henning perhaps we can talk a bit 
on IRC as I get some strange errors, if you have a bit of time)
- decide about some parts of the documentation if we keep it or not. 
docbook xml tools are more advanced than sgml ones, more information is 
displayed now: e.g., svn revision info appears in each document now, 
should we keep it? It is not really what one will expect from revision 
part of a document (should show changes done in the document)

As now, there are few other sgml documents. The templates for module 
documentation. Should we keep them (doc/templates/module)? I think they 
were not in use for quite some time, nor updated. The tls doc will be 
migrated xml (tls/doc). The rest should be now all over xml.

Cheers,
Daniel