Switch use of strtok() to gd_strtok_r()

Switch use of strtok() to gd_strtok_r()

strtok() is not thread safe, so this will potentially break in
very bad ways if used in ZTS mode.

I'm not sure why gd_strtok_r() exists since it seems to do the
same thing as strtok_r(), but I'll assume it's a portability
decision and do as the Romans do.