Add support for Mike Douglas' FDC+ 8MB disk images

[deltecent]

This PR adds support for Mike Douglas' FDC+ 8MB disk images to the Altair disk controller:

The disk image “DISK17.DSK” is 56K CP/M 2.2 which supports 8Mb drives on drives A and B along with standard Altair floppy drives on drives C and D. The 8Mb drive looks like a standard Altair floppy drive but with 2048 tracks instead of 77 tracks.

CP/M is also patched to make the "user" construct of CP/M 2.2 more user-friendly (pun intended) to help organize the runaway file collection that tends to develop on a CP/M hard disk. The update provides the following specific features:

1. The current user number shows in the CP/M prompt (e.g., "A0>" or "A5>", or "B12>").
2. If a typed command is not found on the default user/drive combination, then drive A for the current user is searched, then drive A for user 0 is searched. By making drive A user 0 the repository for system files, most every command will run as expected even when in a different user/drive combination.