strongswan: move strongswan.conf generation to its own function

strongswan: move strongswan.conf generation to its own function

The cross-file handling of variables that are populated in 'config_ipsec' but
then written to 'strongswan.conf' in the 'do postamble' section is prone
to errors, as variables are often overlooked. The config file
'strongswan.conf' is now generated in a single function without the need to
use global variables.

This commit does not change the function and is a preparatory commit.

Signed-off-by: Florian Eckert <redacted>