Get rid of dependencies on native xml+docbook and w3m, remove docs from packages.

* Validate docbook xml using mvn.
* Build manpages using mvn.
* Remove manpages in text format.
* Use jericho instead of  w3m.
* Fix bug with missing content in UPGRADE.txt.
* Make the text in UPGRADE.txt more readable.
Merge pull request #2256 from nawroth/new-manual2

Get rid of dependencies on native xml+docbook and w3m.