Change RACK dependency on TCPHPTS from build-time to load-time

[jtl]

Summary

At present, RACK requires the TCPHPTS option to run. However, because modules can be moved from machine to machine, this dependency is really best assessed at load time rather than at build time.

Test Plan

When I boot a kernel compiled with the TCPHPTS option, the RACK module loads. When I book a kernel compiled without the TCPHPTS option, the RACK module complains that it is missing the tcphpts prerequisite.