Vector MZOS uses a different sector checksum than the standard
Micropolis checksum used by MDOS, CP/M and OASIS. Automatically
detect the checksum on the disk by default, but allow the user
to force the use of a specific checksum using the
--decoder.micropolis.checksum_type parameter.
* Discard a partial sector at the end of the track.
* Do not seek to the index mark for the first sector.
* Use a 64-bit pattern to match the SYNC.
* If SYNC is found too early, search for a subsequent SYNC.
* While decoding the sector record, enforce the SYNC pattern
and track ID.
The addition of the new tcsetattr call fixes a problem where interrupting
fluxengine during a flux read will leave data in the device and/or Linux's
serial buffers, so that the next invocation of fluxengine will fail similar
to
```
Error: command returned garbage (0x27 != 0x0 with status 0x31)
```
(the specific value differs because it's actually flux data)
Merely changing the existing tcsetattr call to specify TCSAFLUSH was not
enough; moving it after the 200ms pause seems to be enough.
Note that it doesn't seem feasible in our USB stack to make DTR reset
the device, since that would take down the USB stack and require a fresh
USB connection.
The addition of the special case for `rlen == 0` in read is for when the
GW-compatible board is reset or crashes and usb-disconnects during a
reading operation. Without this change, fluxengine spins forever at 100%
of a CPU, repeating a read(). After the change, this will cause
the host computer to print messages like:
```
2.1: 200 ms in 68928 bytes
3.0: Error: serial read returned no data (device removed?)
```
and exit.
I only tested these changes on Linux (Debian with kernel 5.10) and in
particular I don't know if/how it will work on a Mac.
