debian: use debhelper compat level 11 with a fallback to compat level 9

The dh_installsystemd command is only available with Debhelper v11 and
higher.  So explicitly declare that we need debhelper 11, and create a
fallback for debhelper compat level 9 (so we can easily backport to
Debian Stretch and Ubuntu 16.04).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
4 files changed