Make CppAsString2() more visible in c.h.

Make CppAsString2() more visible in c.h.

For some reason this standard C string-processing hack was buried in an
NLS-related section of c.h.  Put it beside CppAsString() so that people
are more likely to find it and not be tempted to reinvent local copies,
as I nearly did.  And provide a more helpful comment, too.