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

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.

Reviewed by: rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D15756