Tokenize, like strtok, but reentrant

	[[tokenize(s, set, next)]] takes three arguments: a string
	[[s], a set of characters [[set]] and a string pointer
	[[next]]. It returns a pointer to the first `token' in the
	string [[s]], where a token is a non-empty substring that does
	not contain any characters from [[set]]. To get the next
	token, pass [[next]] instead of [[s]] and repeat this until
	the function returns [[NULL]], signifying that there are no
	more tokens.

	[[tokenize]] permanently modifies the string [[s]]. The
	returned pointers point into the space occupied by [[s]].

<<*>>=
#include <config.h>


EXPORT char *tokenize(char *s, char *set, char **next)
{
    if (!s) return NULL;
    s += strspn(s, set);
    *next = s + strcspn(s, set);
    if (**next != '\0') *((*next)++) = '\0'; else *next = NULL;
    return *s ? s : NULL;
}