So far, one has to set the "fork" value to both the start and stop
goals. Since they have the same name, sharing them in a global
configuration element does the trick. However, the plugin also supports
auto-detection of the fork value according to other parameters:
typically if an agent or jvm arguments are set, forking will be
automatically enabled. This is a problem since the stop goal is not aware
of that.

This commit transmits the value in a property attached to the
`MavenProject`. That way, the stop goal can retrieve that value and
apply the same defaults. This has the side effect that specifying the
fork value isn't necessary anymore.

Closes gh-6747