gnulib-tool.py: Simplify creation of module shell ids. * pygnulib/GLModuleSystem.py (GLModule.shell_id_chars): Remove class variable. (GLModule.shell_id_pattern): New class variable. (GLModule.getShellId): New function. (GLModule.getShellFunc, GLModule.getShellVar) (GLModule.getConditionalName): Use it.