According to Mike's mail, some icedove extensions didn't build with
xulrunner-dev.

This reverts commit 13b57a30.

Lintian: spelling-error-in-copyright explicitely explicitly

Lintian: copyright-refers-to-versionless-license-file usr/share/common-licenses/GPL

Lintian: out-of-date-standards-version 3.8.0 (current is 3.8.3)

filched from iceape 2.0

Lintian: debug-package-should-be-priority-extra,
         wrong-section-according-to-package-name

Lintian: debhelper-but-no-misc-depends