src/libadacl.c: add a special case for the path "." as an argument.

There's a bug (exposed by the most recent test case) in the way the
path "." is handled. Specifically, the dirname() function miscomputes
its parent path as ".", which is clearly not correct.

In this commit, a special case is added for the path ".", and the test
suite passes once more. The implementation is a bit of a hack, however,
and will be improved once the same issue with ".." has been dealt with.