ON-16155: Standardise onload_tool on useradd

Debian `adduser` perl script now wraps common shadow utils
`useradd`, so just call it directly for consistent
group option handling.

scripts/onload_tool

@@ -181,18 +181,10 @@ cmd_set_cplane_user() {
     systemd-sysusers onload.conf
   else
     getent group "$ONLOAD_CPLANE_USER" >/dev/null || groupadd -r "$ONLOAD_CPLANE_USER"
-    getent passwd "$ONLOAD_CPLANE_USER" >/dev/null && exit 0
-
     # We do not require this home directory to exist
-    if command -v adduser; then
-      try adduser --system --no-create-home --home /run/openonload --group "$ONLOAD_CPLANE_USER" \
-                  --shell /usr/sbin/nologin "$ONLOAD_CPLANE_USER" >/dev/null
-    elif command -v useradd; then
-      try useradd --system --home /run/openonload --group "$ONLOAD_CPLANE_USER" \
+    getent passwd "$ONLOAD_CPLANE_USER" >/dev/null || \
+      try useradd --system --home /run/openonload --gid "$ONLOAD_CPLANE_USER" \
                   --shell /usr/sbin/nologin "$ONLOAD_CPLANE_USER" >/dev/null
-    else
-      fail "ERROR: Shadow utils not found. The user $ONLOAD_CPLANE_USER needs adding manually."
-    fi
     exit 0
   fi
 }