util: Const version of strtok_len

Because of limitations in the C type system, we can't a strtok_len
that can work on both const string and non-const strings.  The C
library solves this by taking a const char* and returning a char*
in functions like this (e.g., strchr), but that's not const-safe.
Solve it by introducing strtok_len_c, a version of strtok_len for
const strings.