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.