tun: Add tun.staged.tx and tun.staged.inline_below

tx: false keeps transmit inline and stages only receive. inline_below
sets the idle-reader shortcut threshold, 0 hands every batch to the
workers and the ordered sender or writer.

examples/config.yml

@@ -284,14 +284,18 @@ tun:
   # Staged reservations may use half the window, the other half absorbs inline and control traffic, relay forwarding
   # and network reordering; unlimited forwarding or delay can still exceed it. Waiting for credit on a busy tunnel
   # also delays the other tunnels of the same read, and readers are not guaranteed fairness.
-  # Idle batches below 256 TX segments or RX packets stay on the reader. With multiple routines, those sends still
-  # take tunnel reservation credit.
+  # Idle batches below inline_below TX segments or RX packets stay on the reader, default 256. With multiple routines,
+  # those sends still take tunnel reservation credit. 0 hands every batch to the workers and the ordered sender or
+  # writer; only TX reads too large for a tunnel's whole credit are still sent by the reader, after it drains.
+  # tx: false keeps transmit inline and stages only receive.
   # tx_slots and rx_slots bound the batches each routine has in flight per direction, minimum 2. A TX slot holds one
   # tun read and its ciphertext, about 1.1 MiB; the default is 8, or workers + 2 if more. An RX slot holds one
   # recvmmsg batch, listen.batch * 64 KiB with udp_offloads or * 9 KiB without; the default keeps up to 384 receive
   # entries in flight in 4 to 6 slots: 6 slots or 24 MiB at the default listen.batch of 64. Lower them to save memory.
   #staged:
   #  workers: 0
+  #  tx: true
+  #  inline_below: 256
   #  tx_slots: 8
   #  rx_slots: 6