MFC r359689: config(8): "fix" a couple of buffer overflows

Recently added/changed lines in various kernel configs have caused some
buffer overflows that went undetected. These were detected with a config
built using -fno-common as these line buffers smashed one of our arrays,
then further triaged with ASAN.

Double the sizes; this is really not a great fix, but addresses the
immediate need until someone rewrites config. While here, add some bounds
checking so that we don't need to detect this by random bus errors or other
weird failures.