Give XDG_DATA_HOME priority over XDG_DATA_DIRS for servicedirs

Give XDG_DATA_HOME priority over XDG_DATA_DIRS for servicedirs

According to the XDG Base Directory Specification, “the base directory
defined by $XDG_DATA_HOME is considered more important than any of the
base directories defined by $XDG_DATA_DIRS.”  This makes it easier to
override a system service file as a normal user.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34496
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>