CMake: Add more/better GNUInstallDirs variables usage

1. Use more of GNUInstallDirs' installation location variables: BINDIR
2. Don't assume that prepending CMAKE_INSTALL_PREFIX to any of the location variables makes sense. There is no guarantee that install locations are relative to the prefix (see CMAKE_INSTALL_<location> documentation) - forcefully prepending the prefix to them can generate nonsense. CMAKE_INSTALL_FULL_<location> is what should be used to get a guaranteed-absolute version of the locations.

There are more install location assumption in the systemd services hidden inside debian/, but I'm not sure what to do with them. Some of the normally-installed dbus service files have SystemdService entries that reference them, so I would assume they're intended to be part of a regular installation? But they're kept outside of CMake's control and installed via Debian packaging code, so they require manual installation & are weirder to configure using CMake variables.

Is there any reason those services are handled in that way? Otherwise I'd like to submit a follow-up MR to move them out of debian/, configure_file() them with CMAKE_INSTALL_FULL_BINDIR, and install() them as part of a regular installation - maybe with an option() to opt out of this if systemd stuff is not desired. Any opinions on that?