lnutil.ImportedChannelBackupStorage: change ser format: int16->uint16

In the binary serialised format, replace all instances of int16 with uint16.
In particular, this allows port>32767.
Fixes https://github.com/spesmilo/electrum/issues/8264

I think this is backwards compatible, as in, any existing channel backup already out there,
should be properly parsed with the new code. (new code however can serialise cbs that old
code deserialises incorrectly)

```
>>> struct.pack('<h', 258)
b'\x02\x01'
>>> struct.pack('<H', 258)
b'\x02\x01'
```

electrum/lnutil.py

@@ -264,27 +264,27 @@ class ImportedChannelBackupStorage(ChannelBackupStorage):
 
     def to_bytes(self) -> bytes:
         vds = BCDataStream()
-        vds.write_int16(CHANNEL_BACKUP_VERSION)
+        vds.write_uint16(CHANNEL_BACKUP_VERSION)
         vds.write_boolean(self.is_initiator)
         vds.write_bytes(self.privkey, 32)
         vds.write_bytes(self.channel_seed, 32)
         vds.write_bytes(self.node_id, 33)
         vds.write_bytes(bfh(self.funding_txid), 32)
-        vds.write_int16(self.funding_index)
+        vds.write_uint16(self.funding_index)
         vds.write_string(self.funding_address)
         vds.write_bytes(self.remote_payment_pubkey, 33)
         vds.write_bytes(self.remote_revocation_pubkey, 33)
-        vds.write_int16(self.local_delay)
-        vds.write_int16(self.remote_delay)
+        vds.write_uint16(self.local_delay)
+        vds.write_uint16(self.remote_delay)
         vds.write_string(self.host)
-        vds.write_int16(self.port)
+        vds.write_uint16(self.port)
         return bytes(vds.input)
 
     @staticmethod
     def from_bytes(s: bytes) -> "ImportedChannelBackupStorage":
         vds = BCDataStream()
         vds.write(s)
-        version = vds.read_int16()
+        version = vds.read_uint16()
         if version != CHANNEL_BACKUP_VERSION:
             raise Exception(f"unknown version for channel backup: {version}")
         return ImportedChannelBackupStorage(
@@ -293,14 +293,14 @@ class ImportedChannelBackupStorage(ChannelBackupStorage):
             channel_seed=vds.read_bytes(32),
             node_id=vds.read_bytes(33),
             funding_txid=vds.read_bytes(32).hex(),
-            funding_index=vds.read_int16(),
+            funding_index=vds.read_uint16(),
             funding_address=vds.read_string(),
             remote_payment_pubkey=vds.read_bytes(33),
             remote_revocation_pubkey=vds.read_bytes(33),
-            local_delay=vds.read_int16(),
-            remote_delay=vds.read_int16(),
+            local_delay=vds.read_uint16(),
+            remote_delay=vds.read_uint16(),
             host=vds.read_string(),
-            port=vds.read_int16(),
+            port=vds.read_uint16(),
         )
 
     @staticmethod