VK1ET : Use error corrected DF17

Actually use the (DF17) messages where we do bit correction.

Other changes to AVR message I/O.
1) treat mlat timestamp of zero as invalid (easier to work with than
(-1) and equally unlikely).
2) If we don't have a valid timestamp send a without timestamp
'*.......' message rather than '@.......' with invalid time.
3) Drop interpretation of obsolete AVR ' #' & '$' formats - they
wouldn't have worked anyway (wrong length).