Tor should lock its HiddenServiceDirs

We added a simple string comparison to keep me^Wusers from shooting themselves in their feet by specifying two hidden services with the same HiddenServiceDir in the same torrc, but someday we should also put a lock file in every HiddenServiceDir to prevent users from shooting themselves in their feet in more elaborate ways (e.g. using symlinks, or shared network filesystems, or multiple Tor instances).

The lock file will need to be named something other than lock (perhaps hslock), so that this won't break currently valid (but imprudent) configurations that use Tor's DataDirectory as the HiddenServiceDir for a hidden service.