Insure receive buffer is purged before sending ZRPOS.

Also, zmodem_send_raw() was saving the "last sent" char, even if it wasn't successfully sent.

Eliminated the "errors" mode of zmodem_recv_header_raw() since was no longer used.
Eliminated zmodem_recv_header_and_check() since it's no longer used.

Some debug-log fix-ups (e.g. with progress/offset).