Provide single-port connection to kernels

[fperez]

@minrk, we'd discussed this in the past, so I'll just make an issue here so we can track progress and ideas. I'm finding myself so often connecting to existing kernels in many ways, that I think it really would be useful if we could replace our current

--existing --shell=12663 --iopub=14174 --stdin=16920 --hb=58880

with just something like:

--existing 12345

and have that be sufficient for the rest of the connections to 'just work'.

[minrk]

When we add connection info as files, that will be the only info you need - zero ports, one file. This is already necessary to deal with existing security issues.

[fperez]

Sounds good, we can close this one when that's done. Thanks!

[minrk]

Just for tracking, #688 will be closed when this is done as well.