Fix return code handling of config_testcd() + slightly refactor code

Fix return code handling of config_testcd() + slightly refactor code

The usage of a variable inside a *sub*shell ("RC=$?") breaks
checking for this variable later in the code ("if $RC -ne 0 ..."),
so testcd didn't do what we expect it to do.

While at it slightly refactor the code to match the look'n'feel
of grml-autoconfig.