strongswan: add one default named ipsec uci section 'globals'

strongswan: add one default named ipsec uci section 'globals'

The UCI section 'ipsec' should only be available once, as these are
general settings for 'strongswan.conf'. It makes no sense to configure them
more than once. To ensure that the general settings can also be configured
via LuCI, this section must be present at least once.

For configurations already in the field, a uci-default script is added
that assigns the name 'globals' to the last ipsec section type.

Signed-off-by: Florian Eckert <redacted>