Packages: use groupadd/useradd on Debian-based operating systems.

addgroup/adduser will no longer be installed by default in the
"minbase".  Also, moving to lower-level utilities saves us one runtime
dependency.

pkg/deb/debian/unit.postinst

@@ -25,17 +25,16 @@ BANNER
 fi
 
 if ! getent group unit >/dev/null; then
-    addgroup --system unit >/dev/null
+    groupadd --system unit >/dev/null
 fi
 
 if ! getent passwd unit >/dev/null; then
-    adduser \
+    useradd \
       --system \
-      --disabled-login \
+      --gid unit \
-      --ingroup unit \
       --no-create-home \
       --home /nonexistent \
-      --gecos "unit user" \
+      --comment "unit user" \
       --shell /bin/false \
       unit >/dev/null
 fi

pkg/docker/template.Dockerfile

@@ -58,14 +58,13 @@ RUN set -ex \
     && @@RUN@@ \
     && mkdir -p /var/lib/unit/ \
     && mkdir /docker-entrypoint.d/ \
-    && addgroup --system unit \
+    && groupadd --system unit \
-    && adduser \
+    && useradd \
          --system \
-         --disabled-login \
+         --gid unit \
-         --ingroup unit \
          --no-create-home \
          --home /nonexistent \
-         --gecos "unit user" \
+         --comment "unit user" \
          --shell /bin/false \
          unit \
     && apt-get update \