hook up Challenge for Post-Auth-Type for inner tunnel processing

[jimdigriz]

For inner-tunnel processing it is not possible to use Post-Auth-Type Challenge, for example with rlm_detail, as it is completely skipped.

This PR hooks up the doll to allow this.

Works For Me(tm)

[alandekok]

I think this looks good. Since there's no existing Post-Auth Challenge section, this new functionality shouldn't affect existing systems.

[mcnewton]

LGTM; pulled in manually, thanks.