410user/inc/thread.h should not include thr_internals.h (at least for PSU)

the psu handout doesn't have this include, and if a studence puts for example "include thread.h" in their e.g. cond_type.h, that can cause a circular include cycle and break the build (even tho it would succeed in psu's basecode).

i tried just removing it but it makes libthrgrp fail to build (didn't quite figure out why), so maybe like psu doesn't even include 