[BSD] Primary thread's stack size is reported incorrectly

At least on Mac OS 10.12 we have observed stack sizes of the primary thread not aligned to pages boundries. This can be provoked by e.g. setrlimit() (ulimit -s xxxx in the shell).This voids the computation of the addresses of the guard pages.

Fix:
Apparently Mac OS actually rounds upwards to next multiple of page size however, it is conservative to round downwards here to be on the safe side.