Implement mhng-{mtn,mtp}

These two move the message pointer around after removing the current
message, just like the old version.

Note that this new implementation is a whole lot more robust because
this stuff is all in binary as opposed to before where it was just a
shell script.
Remove the old mtn implementation
Implement mhng-folders

This one is pretty simple, it just lists the current set of folders
along with a few bits of information about them.  I believe I only
bothered to implement this for the original MHng because it was the
easiest thing to implement... :).
Remove "folders", it's obselete