CP/M: Search other user-numbers (and drives?) for include files

[Kroc]

Since we don't have sub-directories in CP/M we could separate core vs. platform vs. library files by user-number (or drive?), so the include directive (.i) should search the other user-numbers if a file can't be found in the current one. A common pattern on CP/M is to store the main program in user 0 so that it's callable from any other user-number. This goes hand-in-hand with adding support for drive-letters [and user-numbers] in file names on CP/M (#9).