Add restart-services to osx

Adds a simple script to allow users to optionally restart the services changed by running `osx/set-defaults` so that they don't have to do a full system restart to see the new defaults take effect.

The script can be executed via `bin/restart-services`, or by choice once `bin/set-defaults` has finished running. `osx/set-defaults` runs as it did before, so `script/bootstrap` and `bin/dot` are not changed by this new script (to avoid having the Terminal killed before they finish running).