Bug#1101528: xfce4-session: bashism in xflock4 breaks locking screen (arithmetic expression: expecting primary)

["Benjamin Kaduk" <kaduk@mit.edu>]

Note that the upstream change switches back (see #1093378, #1092334,
#1064099) to calling dbus-send rather than gdbus and so we might be able to
drop the recommendation on libglib2.0-bin.

-Ben