ArgGroups with validate=false should be non-exclusive

[remkop]

(Related to #810)

If an Argument Group is defined as @ArgGroup(validate = false), it should automatically be set to exclusive = false.

This prevents issues where a new group instance is created for each option in the group. This is not necessary for non-validating groups.

For example:

@ArgGroup(validate = false, exclusive = true, heading = "...%n")
MyGroup group = new MyGroup();

static class MyGroup {
    @Option(names = "-a", required = true)
    boolean a;

    @Option(names = "-b", required = true)
    boolean b;
}

When the user specifies -a -b, the application expects to be able to inspect the group and see both the a and the b field set to true. For exclusive groups this won't happen. While parsing, when the second option of an exclusive group is matched, a new instance of the group is created to prevent having mutually exclusive options in the same group. So, in the above example, the group field would be reassigned a new instance of MyGroup, and only the b field in that instance would be true.

The fix is to automatically set groups to exclusive = false when the group is defined as non-validating.