strongswan: make the include's in the .conf files persistent

strongswan: make the include's in the .conf files persistent

Having scripts diddle user written config files seems potentially
dangerous.  Plus there's really no downside to including some
empty files.  Best to just make the includes be permanent.

Additional feature suggested by Luiz: if a -opkg version of the
config file was created unnecessarily, remove it as part of the
upgrade process since changes won't be happening to that file
as an artifact of the service starting.  The include lines are
now permanent, which means that (1) additional configuration
synthesized by UCI won't be anywhere that opkg (or sysupgrade,
for that matter) cares about since it won't be persistent, and
(2) if changes are being made, then they're being done by a
person with an editor and they really should be distinguished.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
(cherry picked from commit 643df01275798dc4a44d33ac1a8b630c91a80c8c)