Do not allow SBuf::toLower/toUpper() return a value.

Do not allow SBuf::toLower/toUpper() return a value.

Some callers think toLower() method changes the string and some think it
returns a changed value without changing the string. Without unportable hacks
like __attribute__((warn_unused_result)), some of the callers will guess wrong
and not know about it. It is easier/safer to change the object itself and
return nothing.

Provided ToUpper/ToLower() functions (not methods!) that preserve their
paramter and return a new object for callers that need the old functionality.
These functions are a good candidate for __attribute__ hacks.