Purge early output files writability validations

Purge early output files writability validations

Fort used to clear the --output.roa and --output.bgpsec files to make
sure they were writable, during early validations.

So this is why the files spent so much time being empty! This was not
acceptable. It didn't even guarantee the files would still remain
writable by the time Fort needed to properly populate them.

Adjacent progress for #124.