strongswan: migration and merge local_nat into local_subnet

strongswan: migration and merge local_nat into local_subnet

The UCI option 'local_nat' was a misleading legacy name for the
option that actually specifies the local traffic selector(s) used
by strongSwan/swanctl for a connection, i.e. the local subnet(s)
matched by the IPsec SA. It has nothing to do with NAT-T or NAT
detection, and the name repeatedly caused confusion when writing
or reviewing configs.

Merge it to 'local_subnet' to match its actual purpose and to
align with the naming already used for the corresponding
'local_subnet' option and with swanctl.conf's own 'local_ts/remote_ts'
terminology.

Add a uci-defaults migration script so existing installations are
upgraded automatically. It reads any existing 'local_nat' values
(whether stored as a plain option or as a list) and rewrites them
as a 'local_subnet' list.

Signed-off-by: Florian Eckert <redacted>