Const-ify the arguments of str_tolower() and friends to suppress compile

Const-ify the arguments of str_tolower() and friends to suppress compile
warnings.  Clean up various unneeded cruft that was left behind after
creating those routines.  Introduce some convenience functions str_tolower_z
etc to eliminate tedious and error-prone double arguments in formatting.c.
(Currently there seems no need to export the latter, but maybe reconsider
this later.)