procctl(2): Add PROC_WXMAP_CTL/STATUS

It allows to override kern.elf{32,64}.allow_wx on per-process basis.
In particular, it makes it possible to run binaries without PT_GNU_STACK
and without elfctl note while allow_wx = 0.

Reviewed by: brooks, emaste, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31779