Make redis use const pointers where possible

[petdance]

I submitted PR #6302 last night to make functions return const char * when returning literal strings. This should prevent any inadvertant illegal modification of strings.

My next steps would be to start working on making pointer arguments to functions be const pointers where possible. Most places in redis use const pointers, but there are places that don't. I'm hoping that updating those places that don't will make code safer and let the compiler more accurately track uninitialized variables.

Is this something you'd be interested in?