config: Add no-ctfconvert support.

Bump CONFIGVERS to 600018 for this support.

Some files may purposely have debug info disabled or are *source files*
that attempt to run ctfconvert on them. Currently ctfconvert ignores
these errors but I have a change to make the errors real so we can
catch real problems like exceeding type limits.

Sponsored by: Dell EMC
Reviewed by: imp, cem, kevans
Differential Revision: https://reviews.freebsd.org/D24535