strongswan: preserve changed configuration files

[lowjoel]

After reinstalling the packages with the preserved configuration files after a sysupgrade, the reinstalled package config files overwrite what is on disk rather than being placed as conf-opkg. Defining these config files will preserve them appropriately.

Maintainer: @pprindeville @Thermi
Compile tested: aarch64, Linksys E8450, OpenWrt 24.10.0
Run tested: aarch64, Linksys E8450, OpenWrt 24.10.0

Description:
This adds the config files to the conffiles target which should signal to opkg (and apk in future) the config files to be left alone if they exist.

I have checked that the resulting opkg package's control.tar.gz has a new conffiles metadata file. Installing the package now shows this message:

# opkg install /tmp/strongswan-mod-dhcp_5.9.14-r7_aarch64_cortex-a53.ipk
Upgrading strongswan-mod-dhcp on root from 5.9.14-r6 to 5.9.14-r7...
Configuring strongswan-mod-dhcp.
Collected errors:
 * resolve_conffiles: Existing conffile /etc/strongswan.d/charon/dhcp.conf is different from the conffile in the new package. The new conffile will be placed at /etc/strongswan.d/charon/dhcp.conf-opkg.