usermanual: put each instance of <programlisting> into individual <para> environment

needed to get xml2po working
Merge branch 'master' into usermanual

Conflicts:
	doc/usermanual/lighttable/panels/export.xml
	doc/usermanual/lua/lua.xml
	doc/usermanual/tethering/troubleshoot.xml
	doc/usermanual/topics/opencl.xml