Replace add-shell/remove-shell with declarative shells.d trigger

Johannes Schauer Marin Rodriguesrequested to merge
josch/dash:shells.d into debian/unstable

Requires debianutils (>= 5.6-0.1)

For details see https://bugs.debian.org/990440

Closes: #1004172

Edited by Johannes Schauer Marin Rodrigues

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