strongswan: rename 'remote_gateway' to 'remote_addrs'

strongswan: rename 'remote_gateway' to 'remote_addrs'

Rename the 'remote_gateway' UCI option to 'remote_addrs' to match
the 'swanctl.conf' 'remote_addrs' directive, which accepts one or
more peer addresses rather than a single gateway. The previous
name implied a single value and did not reflect that multiple
remote addresses can be configured for a connection.

Store the option as a UCI list to allow specifying multiple
remote addresses, consistent with how 'local_subnet' and
'remote_subnet' are already handled.

Add a uci-defaults migration script to convert existing
'remote_gateway' entries (whether stored as a plain option or as
a list) to a 'remote_addrs' list on upgrade.

If the 'remote_addrs' option is empty, strongswan implicitly uses the value
'%any'. In the `swanctl.conf` documentation, '%any' is already the default
value for 'remote_addrs' if the directive is not specified at all.

Signed-off-by: Florian Eckert <redacted>