Support (and set) launchd user domain target

[MikeMcQuaid]

When running brew services over ssh or sudo as a non-root user the gui domain target for launchd does not work as expected. This produces confusing failures for users.

It was tried before to detect if this was needed from the .plist or changing globally but both approaches were unsuccessful/error-prone.

Instead, use the user domain automatically on the cases we know it's needed: when running over ssh or through sudo. To make clear to users what's happening in these cases: output a warning (which can be hidden with an output environment variable).

For this to work, launchctl list is no longer sufficient. The output here, even when run as root, does not properly list user domain services. Instead, we need to use launchctl print to correctly query the status of these services. This also seems to correctly handle some launchd edge-cases where launched services cannot be detected and lets brew services now stop them.

While we're here, fix some related issues I came upon while working on this:

• improve the brew services command documentation to note it now runs on Linux/systemd too
• use named_args and remove the custom_plist deprecation: it's been long enough and this cleans up the code nicely.
• if a service is status :other (an edge-case that shouldn't be possible in normal operation): actually output this rather than failing with a nil error
• avoid running launchctl list multiple times when unnecessary
• brew services --debug now outputs the raw output from launchctl or systemctl to aid debugging/development
• cleanup some code style and avoid use of plist?
• add System.systemctl_args to avoid repeating System.systemctl and System.systemctl_scope in every call site

---

Depends on Homebrew/brew#16041 to detect sudo or ssh correctly.