Re-introduce ipsec-tools package

Package ipsec-tools was abandoned in oldpackages and hadn't seen much attention recently. I make extensive use of this package and am willing to maintain it within openwrt. Note that I also maintain this package for Debian.

This pull request re-introduces the ipsec-tools package, updates it to the latest upstream release 0.8.2, and works around a configure test that is currently failing in the openwrt build environment. I reported this some time ago in https://dev.openwrt.org/ticket/14141