Use DBusString for all relocation and install-root code

Use DBusString for all relocation and install-root code

This means we handle OOM correctly, and makes it obvious
that we are not overflowing buffers. This change does not
affect the actual content of the strings.

Instead of redefining DBUS_DATADIR to be a function call
(which hides the fact that DBUS_DATADIR is used),
this patch makes each use explicit: DBUS_DATADIR
is always the #define from configure or cmake, before
replacing the prefix.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83539
Tested-by: Ralf Habacker <ralf.habacker@freenet.de>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>