Refactor single instance checks, cleanups and option passing.
Try to avoid races by not trying to create the single instance
lockfile until the socket is ready to receive clients.
Add option to print agent details in Bourne shell format.
Change single instance lock file format to include pid.
Re-assign -s to Bourne shell format, --socket-path doesn't need a short option.
Minor refactorings.
Add option to execute child process and exit when it exits.
Forward nominally fatal signals to the child instead of exiting.
Add option to daemonise process.
If used with a command to execute, run the command in the parent process,
and fork off the agent proxy as the daemon.
Add help text for daemonise and execute.
Add option no-recurse, use existing socket if it looks like sshod.