lualoader: Fix loader.conf(5) EOL validation for 'exec' lines

This includes some light rework to simplify the line parsing, as well. If
we hit a line match, we'll always either use the line and move on to the
next line, or we'll spew out malformed line errors.

We had multiple spots to output the error and set the status based on
whether we had a non-nil first capture group or failed EOL validation, but
it was always the same error. Light rework entails a small label jump to
skip error handling and elimination of 'found' local.