Call sem_unlink on semaphores before attempting to create them

Due to the lack of uniqueness in the semaphore name, and the fact that the
tests don't have cleanup routines, an interrupted test can leave a semaphore
"laying around", causing all subsequent attempts to run the test to fail

I will file a NetBSD PR for this issue soon
Add reachover Makefiles for contrib/netbsd-tests/lib/librt

A variant of this code has been tested on amd64/i386 for some time by
EMC/Isilon on 10-STABLE/11-CURRENT. It builds on other architectures, but the
code will remain off until it's proven it works on virtual hardware or real
hardware on other architectures

Sponsored by: EMC / Isilon Storage Division