Replace menu with a better command line

[thejpster]

We want:

• built in commands
• executing programs off disk, with arguments
• environment variables - assignment and expansion
• batch files / shell scripts
• redirection to file

The shell will steal ram from the top of the tpa, while still allowing programs to run in what is left. Will need some kind of heap allocator that can do that.

Built in commands:

CLS
DIR [/W] [<pathspec>]
CD [<dir>]
PWD
MKDIR <dir>
RMDIR <pathspec>
DEL <pathspec>
COPY <pathspec> <dir>|<path>
MOVE <pathspec> <dir>|<path>
RENAME <pathspec> <name>
SET [<varname> [= <value>]
HELP [<topic>]
MODE [<sub-commands>]
DATETIME [<iso-8601>]
TYPE [/P] [<pathspec>]

Maybe check back to an MS-DOS 5.0 reference guide.