Switch to package.maintscript

Fixes following lintian warnings:

W: libgl1-mesa-dri: maintainer-script-should-not-use-dpkg-maintscript-helper postinst:4
W: libgl1-mesa-dri: maintainer-script-ignores-errors postinst
W: libgl1-mesa-dri: maintainer-script-should-not-use-dpkg-maintscript-helper postrm:4
W: libgl1-mesa-dri: maintainer-script-ignores-errors postrm
W: libgl1-mesa-dri: maintainer-script-should-not-use-dpkg-maintscript-helper preinst:4
W: libgl1-mesa-dri: maintainer-script-ignores-errors preinst