SdFatInput improvements
SdFatInput improvements

- Automatically write-unmount when the last write-reference is released.
- Add missing serial-output poke calls.
- Allow M22 to start write-unmounting even if the subsequent full-unmounting would not be allowed (error out at that point).

ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter,fixator/aprinter,fixator/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter,fixator/aprinter,ambrop72/aprinter,johnkingsley/aprinter