strongswan: rename 'gateway' to 'remote_gateway'

strongswan: rename 'gateway' to 'remote_gateway'

The UCI option 'gateway' in the ipsec config (consumed by
/etc/init.d/swanctl) is ambiguous. It is not clear from the name
alone whether it refers to the local or the remote IKE/IPsec
peer address, which has repeatedly led to misconfiguration.

Rename it to "remote_gateway" to make the direction explicit and
to match the existing "remote_subnet" naming. If the option is
unset, the init script now falls back to the strongSwan default
of '%any', so connections that accept any remote peer no longer
require an explicit placeholder value.

Add a uci-defaults migration script to rename existing 'gateway'
entries to 'remote_gateway' and remove the value 'any' on upgrade. This
is new default if 'remote_gateway' is not set.

Signed-off-by: Florian Eckert <redacted>