Git Code Merge Request #5: Improvements for environment variables (merged)

- PCOMM: SET command can modify environment
- PCOMM: PROMPT command updates environment
- PCOMM: PATH command updates environment
- PDOS-32 programs get environment at startup
- Child process inherits environment of parent
- PDOS kernel set COMSPEC before starting PCOMM
- PDOS kernel set BOOTDRIVE to boot drive