Implement execv

[thejpster]

MS-DOS has an execv API. This replaces the current (and indeed only) process's executable with a different one, leaving all the file handles as-is. This might be useful for Neotron OS.

See the exec functions.

[thejpster]

Alternatively you can mark file handles with an ID representing the process that started them, using a process stack. New processes can be pushed on to the stack, but the process must exit to be popped off the stack and at that point all its handles can be closed.

See the spawn functions.