[DOS/FAT32] enable selection of auto_tx and the fast write loop via U0>B

[mooinglemur]

• This moves auto_tx to the correct bit of the SPI_CTRL register, also mirroring a recent fix on the emulator side. Now both match hardware
• Changes the hard defines to a variable switch. sd_fast has bit 6 set for fast reads and bit 7 set for fast writes.
• Accessible through DOS"U0>B"+CHR$(n) where n is 0 for traditional, 1 for fast reads, 2 for fast writes, 3 for both
• The default is slow/traditional, as these fast read/write modes have not been well-tested yet. Testers can change the mode by the channel 15 command above.

[jburks]

minor: since fast is hoped to eventually be the default, perhaps the branch should go to slow and fast should be a cycle faster?

[mooinglemur]

The branch would be too far to jump over the fast routine, so nothing would be saved here.

[jburks]

Something for possible consideration: there could be a situation where for some reason the user's system cannot tolerate fast read or write (e.g. the system is overclocked or we enable a slower SPI bus to a Wi-Fi peripheral). It might be good to have a hard override that prevents setting fast reads or writes.

[mooinglemur]

How do you see this working?

For now, the variable gets initialized to 0 at boot (with the most recent push) and the user has to explicitly change it.