execute: sort directory entries when loading credentials recursively

execute: sort directory entries when loading credentials recursively

Given that the recusive credential loading allows two ways to load the
same credentials, it's important to define a clear order so that it is
always the same one that wins.

i.e. if you use LoadCredential=foobar:/tmp/xyz and there are two files
/tmp/xyz/abc/cde and /tmp/xyz/abc_cde these would both result in a
credential foobar_abc_cde being set, hence it is important to make clear
which one shall win, and that it is always the same one.