Convert all tabs to 4 spaces for subroutine _validate_args.
Remove trailing whitespace and canonicalize things according to Perl
Best Practices. This is in anticipation of further changes coming.

Setting this to true will give the web user an underscore "override" checkbox
when creating a record within a domain.

Use extra_argv to pass UI checkbox value to this function to determine
if underscore validation needs to occur.

Pass "allow_underscore_override" value into _validate_args.

If configuration values are set appropriately then return a string and
checkbox for overriding underscore validation.

Potentially alter form with underscore validation override checkbox.