ssh: Support kw_ssh_start_agent() on Mac

Mac lacks flock(1), yet Python 2.7 exposes flock(2) through the fcntl
module. Therefore, on Mac, use a tiny Python script to lock
'~/.ssh/ssh-agent.lock'.

Tags: public
git-archive-ref: Fix for Mac

Replace `date --utc` with `date -u`, because '-u' is portable across
Linux and Mac.

Tags: public