Funder-side channel stuck in CHANNELD_AWAITING_LOCKIN if funding never confirms

When a node is the funder of a channel whose funding tx never
confirms — broadcast rejected at ATMP, evicted from mempool, or
simply never broadcast — the channel record stays in
`CHANNELD_AWAITING_LOCKIN` indefinitely. CLN already implements the
BOLT 2 fundee-side forget rule (PR #1468,
`--max-funding-unconfirmed-blocks`, default 2016) but has no
equivalent on the funder side. Once the would-be funding inputs are
spent in other transactions the funding tx is permanently
unconfirmable, yet the channel record remains.

Similar to #9102

## Concrete instance

More details about the following channel can be found in [clboss
issue #312](https://github.com/ksedgwic/clboss/issues/312).
@ksedgwic operates the funder.

### Channel

- `channel_id`: `0b12c289525bf5593e932eca14277dbcb0921f125731b586d3a75d2f4b328dde`
- `funding_txid`: `de8d324b2f5da7d386b53157121f92b0bc7d2714ca2e933e59f55b5289c2120b` — never confirmed on chain
- opener: us; closer: us
- created 2025-10-14; broadcast rejected by bitcoind at 22:46:28 UTC
with `min relay fee not met, 507 < 511`; spenderp ran cleanup
(`unreserveinputs`) but the channel record on lightningd's side
persisted
- still in `AWAITING_UNILATERAL` (a later close attempt also could not
broadcast — its commitment input is the never-existing funding
output) on our node as of 2026-05-04, ~6.5 months later
- All 7 of the funding tx's input UTXOs are now spent in other
transactions, so the funding tx can never confirm

## Reproducer

Reproduced by #9113