While technically dconf on darwin could work, our activation step
requires dbus, which only *lightly* supports Darwin in general, and not
at all in the way it's packaged in nixpkgs. Because of this, we just
disable dconf for darwin hosts by default.
In the future, if someone gets dbus working, this _could_ be re-enabled,
unclear whether there's actual value in it though.
Specifically, inform the command about the absolute path of
dbus-daemon. Otherwise it will try running dbus-daemon from PATH,
which may not always work.
PR #3405
Before enabling dconf in Home Manager, dconf must be enabled in system
config.
Otherwise it will fail like this:
```
$ home-manager switch
Starting Home Manager activation
Activating checkFilesChanged
Activating checkLinkTargets
Activating writeBoundary
Activating installPackages
replacing old 'home-manager-path'
installing 'home-manager-path'
Activating dconfSettings
error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.
```
This change makes use of the `extend` function inside `lib` to inject
a new `hm` field containing the Home Manager library functions. This
simplifies use of the Home Manager library in the modules and reduces
the risk of accidental infinite recursion.
PR #994
