Add shebang lines to all debian packaging scripts

[danielhodder]

Pretty much the same as #25

Add a shbang using /bin/sh to all the debian scripts. Installing the session-manager-plugin under the docker-buildkit QEMU build emulation fails without this since it tries to interpret the scripts as binaries, which they are not. Adding the shbang line makes the interpreter correctly understand these are shell scripts and run them under the shell, rather than as a binary.

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[encron]

Ran into the same issue during docker build. Had to resort to unpacking the .deb file, adding the shebang lines and then repacking and installing to work around it. Would be great to have this simple fix merged!