Let's try to recognize paths (i.e. those with a "/") as source for
credentials to load, and then read them from the file system. Also, only read
credentials from an inbound credentials directory if the source
qualifies as valid credential name.
Otherwise print a nice error.
We must check the return value of GREEDY_REALLOC for OOM, and the
pointer are updated already on success, hence it's a bad idea to make a
copy of the pointer beforehand.
Mike Yuan
Lennart Poettering
Sam Leonard