strongswan: rename 'local_ip' to 'local_addrs'

strongswan: rename 'local_ip' to 'local_addrs'

Rename the 'local_ip' UCI option to 'local_addrs' to match the
'swanctl.conf' 'local_addrs' directive, which accepts one or more
local addresses rather than implying a single fixed IP. The
previous name suggested only one address could be configured.

Store the option as a UCI list to allow specifying multiple local
addresses, consistent with how 'remote_addrs' is already handled.
If left empty, no 'local_addrs' line is written to 'swanctl.conf',
so strongswan falls back to its own default of '%any'.

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

Signed-off-by: Florian Eckert <redacted>