Somebody started playing with const, so of course the outcome

Somebody started playing with const, so of course the outcome
was cascades of warnings about mismatching const decls.  Overall,
I think const creates lots of headaches and solves almost
nothing.  Added enough consts to shut up the warnings, but
this did require casting away const in one spot too (another
usual outcome of starting down this path):  the function
mymemreplace can't return const char*, but sometimes wants to
return its first argument as-is, which latter must be declared
const char* in order to avoid const warnings at mymemreplace's
call sites.  So, in the case the function wants to return the
first arg, that arg's declared constness must be subverted.