[osx] Use `sudo` for `systemsetup`

Trying to use `systemsetup` without `sudo` generates the following
message: "You need administrator access to run this tool... exiting!".
[shell] Add more settings and link to `.screenrc`

* Add more default settings.
* Include link to the Screen `Command Summary` documentation, namely:
  https://www.gnu.org/software/screen/manual/screen.html#Command-Summary