Create a correct AST node when encountering <filename>
Fix handling of the --module-path and --include-path options

Before this commit, if user didn't specify the --module-path option and
the |Y2R_MODULE_PATH| environment variable wasn't defined, the program
crashed (because we tried ot call |split| on |nil|). Similarly with
--include_path/|Y2R_INCLUDE_PATH|. This commit fixes the problem.
Ignore <typedef>

Typedefs can be completely ignored because ycpc expands defined types in
the XML, so we never actually encounter them.