Debian/Ubuntu: default debhelper compat level increased to 11.

This is the version installed out of the box on Ubuntu 18.04 "bionic"
which is the oldest one from supported distros.

While here:
 - Standards-Version increased to 4.1.4 (matches Ubuntu 18.04 as well),
 - explicitly added systemd service for nginx-debug binary (previously
it was installed by dh_systemd_enable from modified init script).