Reset currentLine when changing state to printing

This keeps gcodePos and current line in sync when changing status to "Printing"
Overhauled resend handling to also work with alwaysSendChecksum feature. Also introduced new feature flag resetLineNumbersWithPrefixedN to make M110 commands send the target line number as part of their N prefix (Repetier), not as a separate N parameter (Marlin & co)