config(1): Fix build of old trees (bin/49389) (take 2)

Look into sys/conf/Makefile.kern.inc to check if "build_kernel" .USE target
is defined.  If not (== old source trees), generate one on the fly.
Revert debug code.  Simplify.
- Don't allow bypassing file size limits with crontabs from stdin.
- Allow signals while reading the user crontab file; doing "crontab -"
  does not let you abort otherwise, and doing ^Z, kill %1 leaves turds
  in /var/cron/tabs