It may be fragile, but it does sort of work.

This is not as generic as having a single 'term' command, but
it's a lot more convenient to use.

Caution: code is incomplete - can't set a default type from the CLI
and in some places the phrase xterm should be replaced by terminal.

This reverts commit 07aad110.

When the semantics are clear, functionality like this will get added back to
Mininet.

Conflicts:

	mininet/node.py

We should check NOXController and RemoteController to make sure that
they are not broken, but I cannot do this at the moment.

Also rearranged parameters in link command to

mininet> link s1 h2 up

To resemble ifconfig a bit more.

It seems easier to rely on node.waiting for the moment.

I've changed the way things work a bit:

1. netns is replaced by mnexec, a general-purpose mininet helper.

2. For interactive commands, we now use mnexec -p, which prints out
   the pid, so we can kill it when someone hits control-C!

3. We close file descriptors for subshells. This might save memory,
   but who knows.

4. We detach our subshells from the tty using mnexec -s; thus
   control-C should not terminate everything.

5. Given 4, mn -c is now necessary if you kill mininet.

Also print warning if user chooses a loglevel that will hide CLI output.

All commands should work now.

There is probably a better way of doing this, but currently
parseCustomFile can modify globals (e.g. TOPOS) as well as
instance variables (self.validate) and classes (e.g. MyTopo),
which are also in the global name space.

Inconveniently enough, lambdas don't seem to be full closures
in Python; if they were, this trickiness would be unnecessary.

Even so, using execfile() seems like it might be a bit dubious...

Shell now works (although we should use quietRun instead to avoid
file descriptor problem!)

exit/quit/EOF now all exit.

Renamed pingall and pingpair to make them easier to type - I think
commands should not require use of the shift key in general, although
ping-all and ping-pair might be OK.

Convert CLI to use Python Cmd object, which provides line editing.

with the exception of topo.py, which I haven't touched.