If * stringp is NULL, the strsep() function returns NULL and does
nothing else. Otherwise, this function finds the first token in the string *
stringp, where tokens are delimited by symbols in the string
delim. This token is terminated with a `\0' character (by overwriting
the delimiter) and * stringp is updated to point past the token. In
case no delimiter was found, the token is taken to be the entire string *
stringp, and * stringp is made NULL.