tcp_hpts: make the module unloadable

Although the HPTS subsytem wasn't initially designed as a loadable
module, now it is so. Make it possible to also unload it, but for
safety reasons hide that under 'kldunload -f'.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D43092

(cherry picked from commit 48b55a7c7be4175998f9b26dfbec5a561acbd936)