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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33704