stand: Allow stand.h to be included in C++ programs

Allow stand.h to be included in C++ programs. This is little more than
using our stylized BEGIN_DECL / END_DECL around the entire
file. There's no run-time support for C++, so the C++ that can be used
is quite limited. It is enough for libunwind, though.

Sponsored by: Netflix
Reviewed by: jrtc27, kevans
Differential Revision: https://reviews.freebsd.org/D37946

(cherry picked from commit 5cf20707ba79b5c06b26381da0013c65004b53be)