[entropy.db.skel] write locking output to stderr

This avoids breaking scriptability of --quiet options
[entropy.db.skel] improve output during lock contention phases
[entropy.const] add /run/entropy directory setup, expose it through etpConst['entropyrundir']
[entropy.db.skel] expose lock_path() and make file lock path configurable
[entropy.db.sqlite] use lock_path() instead of self._db for file locking