Fix: SET DEFAULT TO command paths and working directory

[Irwin1985]

SET DEFAULT TO would throw a directory not found exception when passed pure drive letters (ej: C:), SYS(5) results, or MS-DOS shorthands.

• SET DEFAULT TO now correctly updates the process's working directory (Environment.CurrentDirectory).
• Root prefixes like \ and C:\ ares now protected from being improperly stripped.
• Ensures SYS(5) and SYS(2003) inherenlty stay in sync with the new default path.

[RobertvanderHulst]

Irwin,
In the code inside Set() you are generating an error when the directory does not exist but you are not throwing the error.

[Irwin1985]

Irwin, In the code inside Set() you are generating an error when the directory does not exist but you are not throwing the error.

Thanks Robert! Good catch.