Detect and refuse reentrant event_base_loop() calls

Detect and refuse reentrant event_base_loop() calls

Calling event_base_loop on a base from inside a callback invoked by
that same base, or from two threads at once, has long been a way to
get exceedingly hard-to-diagnose errors.  This patch adds code to
detect such reentrant invocatinos, and exit quickly with a warning
that should explain what went wrong.