ejabberdctl: support concurrent connections with bound conn names

ejabberdctl: support concurrent connections with bound conn names

If flock is available, ejabberdctl will use it to grab one
of a bound number of connection names. This allows concurrent
connections while using a bound number of atoms.

Using PID, timestamps or random strings for transient connection IDs
(which would avoid the need for flock) uses an unbound number of atoms.
This can effectively DoS servers, as these connection names are
not garbage collected.