Add a new system config directive called "nonoptional" which specifies

Add a new system config directive called "nonoptional" which specifies
files based on options which MUST be specified in the kernel config.

Make the option to maintain compatibility with DragonFly 1.2 (and older
1.3 kernels) non-optional.

Older versions of config will not recognize the new option and generate
a reasonable error, rather then blindly compile a kernel without 1.2
support, hopefully prompting people to recompile their config
program rather then post a bug report.