CheckerGccPlugins: fix feature to check entire directory trees
When searching for the ATLAS_CHECK_THREAD_SAFETY
flag file, do not stop
at the package boundary but keep looking for a possible flag file in the
parent directories. Stop once the maximum depth is reached or one of the
"stop directory names".
This restores the feature mentioned in the README that an entire directory tree could be checked. It's a very useful feature to ensure new packages do not go unchecked.
Also update the list of "stop directory names".
Edited by Frank Winklmeier