Update to debhelper-compat 12

Update to debhelper-compat 12

Use the versioned debhelper-compat dependency instead of debian/compat.

Add ${misc:Pre-Depends} for openafs-client and openafs-fileserver that
install init scripts.

Switch to using dh_missing instead of dh_install --fail-missing.

Since openafs uses regen.sh to wrap autoreconf, pass dh --without-autoreconf.

Adapt to separate dh_installinit and dh_installsystemd stages.

Change-Id: I7f3ca2df005c252c4e45e5c929e9da2cf39ebaf5