implement MEM command

[stsp]

While mem is usually a stand-alone program, it would be cool to have it built-in. We can't implement the entire DOS suite, so my idea is to put the essential commands into command.com itself. Also note that MEM is a bit special: in 16bit command.com the internal commands are executed w/o freeing extra memory, while the external ones need a lot of free mem, so command.com frees its mem before calling an external cmd. MEM however needs to always be executed with as much free mem as possible, or it would be useless. But in comcom32 there is no need to free any DOS mem before executing anything. So the need to have MEM as an external cmd doesn't apply here.