Fix glibc-2.43 constness warnings

Fix glibc-2.43 constness warnings

Glibc 2.43 added C23 const-preserving overloads to various string functions,
which change the return type depending on the constness of the argument(s).
Currently this leads to warnings from calls to strtok() or strchr().
Fix this by properly declaring the respective variable types.

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>