Remove listen mode

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.

tests/acceptance/fakes/dest/break_after_hello.rb

@@ -7,7 +7,7 @@
 require 'flexnbd/fake_dest'
 include FlexNBD
 
-addr, port, src_pid, _, _, sock = *ARGV
+addr, port, src_pid, sock = *ARGV
 server = FakeDest.new( addr, port )
 client = server.accept