[CMAKE] Enable DBus on Linux only

Summary:
The current option name is misleading, since only DBus notifications are
disabled and so apply to Linux only. Change the name to better reflect
the behavior and default to off for non-Linux OSes.

Test Plan:

cmake -GNinja .. -DENABLE_DBUS_NOTIFICATIONS=OFF
ninja check

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5679