Allow users to override CSTD/CXXSTD on a per-prog basis

The current logic for CSTD/CXXSTD requires homogenity as far as the
supported C/C++ standards, which is a sensible default. However, when
dealing with differing versions of C++, some code may compile with C++11, but
not C++17 (for instance). So in order to avoid having people convert over their
code to the new standard, give the users the ability to specify the standard on
a per-program basis.

This will allow a user to override the supporting standard for a set of
programs, mixing C++11 with C++14 (for instance).

Reviewed by: asomers
Apprved by: emaste (mentor)
MFC after: 1 month
MFC with: r345708
Differential Revision: https://reviews.freebsd.org/D19738