exec: Introduce the PROC_PS_STRINGS() macro

Rather than fetching the ps_strings address directly from a process'
sysentvec, use this macro. With stack address randomization the
ps_strings address is no longer fixed.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 706f4a81a81250a326ea25914e7effe1768f1a37)