This makes it easier for the tests (and supervisor) to guarantee to be
able to connect to the server socket.
Also this patch moves freeing the mirror supervisor into the server
thread.
Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.
Previously, the --verbose flag was only present in debug builds. Now
it's present whether you define DEBUG or not. What changes is the
amount of information printed to stderr: DEBUG sets the --verbose log
level to 0 (debug), while DEBUG unset sets it to 1 (info). This makes
driving the binary slightly simpler as you don't have to detect whether
it's a debug build by scanning for "--verbose" in the help output.
