4875: Let the OS choose port numbers for fake servers.

4875: Let the OS choose port numbers for fake servers.

Fixes a race condition where test case N+1 can't listen on port 2990
because test case N hasn't shut down its listener.

Also removes the artificial acceptance requirement that nobody else on
the testing host is using the arbitrarily assigned port range
2990..299x.

Incidental changes:

* rename RunBogusKeepServer to RunFakeKeepServer (to match
  RunSomeFakeKeepServers and fix the misleading implication that the
  resulting server does something bogus).

* return a KeepServer object from RunFakeKeepServer (for better parity
  with RunSomeFakeKeepServers).