[libc] Give more functions restrict qualifiers (NFC) (#78061)

strsep, strtok_r, strlcpy, and strlcat take restricted pointers as
parameters.
Add the restrict qualifiers to them.

Sources:
https://man7.org/linux/man-pages/man3/strsep.3.html
https://man7.org/linux/man-pages/man3/strtok_r.3.html
https://man.freebsd.org/cgi/man.cgi?strlcpy