strchrnul() is a GNU extension

[Sven M. Hallberg]

Discovered during porting to OpenBSD:

The function split_args() in gschem/src/x_controlfd.c uses the strchrnul() function which is not available at least on OpenBSD. It would be nice to refactor the code to avoid the need for it, but in the interest of not introducing new bugs the attached patch just replaces strchrnul() itself with an equivalent (if less readable) construct.