Replacement su for SCO Unix

[Guy Harris]

>Not really stupid (i must admit to microsoft). 'shell ?' means
>'shell != 0' and *not* 'shell != NULL'. You should not rely on
>NULL being the same as 0.

...unless you're using a C compiler rather than something that pretends
to be a C compiler.  If your C compiler can't cope with

	if (shell)

and

	if (shell != 0)

and

	if (shell != NULL)

and treat them all the same, it's broken.