On Catalina, the /nix filesystem might not be mounted at start time.
To avoid this service not starting, we need to keep the launch agent
outside of the Nix store. A wait4pid will hold for our /nix dir to be
mounted.

Fixes #3125.

 - darwin installer: delete hardware report, not necessary
 - moves os-specific code from the darwin installer to to `poly_*`
   functions
 - adds profile.d support to the profile targets, which automatically
   handles many distros which don't have a /etc/bashrc but do have an
   /etc/profile.d
 - /bin/bash -> /usr/bin/env bash
 - document why each excluded shellcheck check is excluded
 - rename the multi-user to Daemon-based

The default profile already loads /etc/bashrc.

The installer will error out if a user's shell configuration includes any mention of ~nix-profile~, even if this is in a comment. This change is designed to do the bare minimum to ignore lines beginning with a `#`.

In particular, drop the "build-" and "gc-" prefixes which are
pointless. So now you can say

  nix build --no-sandbox

instead of

  nix build --no-build-use-sandbox

Also output in the status report that the user is very silly

Starve the TTY of input to ensure this works, but provide yes to the
current installer to handle the current broken case.

Don't install a second nix after the initial installation, and the rsync change fixes a bug hidden by the nix replacement where the store files were being owned by the installing user due to rsync's -a implying -og.

relevant directions

Prompt for sudo before validating assumptions, and check ourselves for root-owned files instead of making a scary warning.