add dummy keymode to iterate over all modes

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5289 43c1dc8c-776e-4df2-8ef1-6e829299be21
fixes #155, fix provided by Diederik de Groot;needs some improvements to reduce sofkeySet update

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5290 43c1dc8c-776e-4df2-8ef1-6e829299be21