postinst.d scripts: "source /usr/share/whonix/postinst.d/pre.bsh" instead of having the pre stuff on top of every individual file
whonix_shared/usr/share/whonix/postinst.d/pre.bsh: set WHONIX_DEB_DEBUG to 1, if -x is set
whonix_shared/usr/share/whonix/postinst.d/pre.bsh: non-empty error message "see above" in case -x was set