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
(cherry picked from commit 02c0546033)
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
