Simplify startup and dependencies

Move startup scripts to /etc/s6-overlay/startup.d
Create oneshot service named "startup" which executes all files in
/etc/s6-overlay/startup.d in alphabetical order
Remove various oneshot scripts from longrun service dependencies
Add "startup" oneshot service to longrun service dependencies

rootfs/etc/s6-overlay/s6-rc.d/startup/up

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec /etc/s6-overlay/scripts/startup

rootfs/etc/s6-overlay/scripts/startup

@@ -0,0 +1,18 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash disable=SC1091,SC2076
+
+source /scripts/common
+
+SDIR=/etc/s6-overlay/startup.d
+
+# exit 0 for nonexistent or empty directory
+if ! [[ -d "$SDIR" ]] || [[ -z "$(ls "$SDIR")" ]]; then
+    exit 0
+fi
+
+for NAME in "$SDIR"/*; do
+    if ! s6wrap --quiet --prepend="$NAME" --timestamps --args "$NAME"; then
+        s6wrap --quiet --prepend=startup --timestamps --args echo Error running "$NAME"
+        exit 1
+    fi
+done