ssh-agent: prevent environment file from flapping

On an OS X laptop, the variable `$HOST` changes a lot depending
on what wifi network you're connected to.  This causes a lot
of `~/.ssh/environment-$HOST` files to be created and
causes multiple ssh-agents to created.

Instead, use `scutil --get ComputerName` to get something
more stable.
Merge pull request #2603 from docwhat/ssh-agent-hostname

ssh-agent: prevent environment file from flapping