[arch-general] updated PKGBUILD for openswan

The openswan package in core is rather old. I've updated the PKGBUILD for the latest version, 2.6.23. This includes rather a lot of build changes - some are cleanup that could have happened before, some are new for upstream changes. I built this to investigate whether I could use it instead of strongSwan. I could not get it to work for my purposes, but rather than let that work go to waste, I figured I'd submit the PKGBUILD here.