Return AST_MODULE_LOAD_FAILURE when chan_skinny.so is already loaded, so that chan_sccp.so is not partially loaded, but unloaded instead

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4866 43c1dc8c-776e-4df2-8ef1-6e829299be21
Added .description to module registration, so 'module show line chan_' shows the correct module name

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4867 43c1dc8c-776e-4df2-8ef1-6e829299be21