strtok_r in glibc 2.0.7 bugginess

[Leif Johansson <leifj@matematik.su.se>]

I just built heimdal 0.0t (when, oh when will the
next one come along ;-) on a rh5.2 box and it dumped
core on startup. It looks like strtok_r is buggy
in glibc-2.0.7 (redhat release 29). I tried to replace
it with the roken version. However strtok_r.c was 
missing from lib/roken/Makefile.am. An easy fix.

The original one dumped core on strtok_r("+"," \t",&pos)
i.e when the string does not contain a token.

	Cheers Leif