Add "unknown" value for FatalErrors directive.

[michaelrsweet]

Add a new "unknown" value (CUPSD_FATAL_UNKNOWN in scheduler/conf.h) for FatalErrors that will cause unknown directives to prevent cupsd from starting.

[dhruvDev23]

Hi @michaelrsweet , i want to work on this issue. can u assign it to me?

[michaelrsweet]

@dhruvDev23 No need to assign it, just work on it and open a PR when you have a fix ready.

Thanks!