Support searching multiple databases in plocate.

Support searching multiple databases in plocate.

We support multiple -d arguments, single -d arguments with multiple
databases (colon-separated), and LOCATE_PATH. The two latter should
be compatible with mlocate, although we support escaping colons in
file paths and I believe mlocate does not.

This is slightly trickier than one might expect due to security
considerations; we're not robust against malicious input, so we need to
make sure that if we process an attacker-supplied database, the process
has already dropped privileges and cannot subvert a privileged reader.