rust directory tree and logical names reorganization

[rust11]

rust directory/logical name tree
The root of RUST's development directory has historically been "F:", whether a real disk or a SUBST disk. However, linking trees to device names is not a good idea at all and has long needed correction. However, I have over 500 source RIDER files with RUST logical names that translate to fully qualified specs in the translated .C files (and many thousands when backups are considered). Thus the solution has to remain compatible with the earlier practice.

In fact, it turns out to be simpler than I imagined: I define F: as a logical name:

o define f c:\f