Fix construction of arguments passed to daemons

Summary:
When adding args to daemons with `phd debug` or `phd launch` (like `--no-discovery` for the `pulllocal` daemon), we currently don't get it quite right when passing them from the overseer down to the daemon itself. In particular, we end up with:

  php ./exec_daemon.php DaemonName --trace --custom-arg

...where `--trace` is daemon infrastructure args, and `--custom-arg` is intended for the daemon itself. The correct invocation is:

  php ./exec_daemon.php DaemonName --trace -- --custom-arg

...that is, a `--` between arguments the daemon wrapper is expected to understand and completely custom arguments intended for the daemon itself.

Make the overseer build such a string.

Test Plan: Ran `bin/phd debug pull -- --no-discovery`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6725