Fix handling of duplicate initial `ChannelMonitor` writing

Fix handling of duplicate initial `ChannelMonitor` writing

In e06484b0f44155e647ff29810d2f187967e45813, we added specific
handling for outbound-channel initial monitor updates failing -
in such a case we have a counterparty who tried to open a second
channel with the same funding info we just gave them, causing us
to force-close our outbound channel as it shows up as
duplicate-funding. Its largely harmless as it leads to a spurious
force-closure of a channel with a peer doing something absurd,
however it causes the `full_stack_target` fuzzer to fail.

Sadly, in 574c77e7bc95fd8dea5a8058b6b35996cc99db8d, as we were
dropping handling of `PermanentFailure` handling for updates, we
accidentally dropped handling for initial updates as well.

Here we fix the issue (again) and add a test.