Motivation:

BTrace agent currently sets the port number given by a user to
`btrace.port` system property as-is. This behavior can be a problem
when:

- The agent actually failed to bind to the given port, because a user
  may still attempt to connect to the port number specified in
  `btrace.port`, where other server may be running already there.
- A user cannot know the actual port where BTrace agent is running when
  the user specified `0` as the port number, because we only know the
  actual port number *after* creating a server socket on it.

Modifications:

- Set the actual local port number after creating a `ServerSocket` so
  that a user can later figure out the actual port number even if the
  user specified `0` as the port number.

Result:

- A user can know the actual local port number, even when the user
  specified `0`.
- A user can know the agent server socket is not open easily by checking
  the existence of `btrace.port` System property.