Improve test_fork_exec to only have a single attempt and cater for slow interpreters like pypy. Fixes #25.
Rename reinstall_bind_delay to reinstall_delay

"reinstall_bind_delay" describes correctly the underlying implementation,
but the manhole api should not expose implementation details.  From the
user point of view, this the delay before a new manhole is reinstalled
in a child process after fork, therefore the name is "reinstall_delay".