No hardcoded sysconfdir

These patches allow strongswan to run without any configuration in a global location (e.g. `/etc`).

In particular, this is useful for the [NixOS](http://nixos.org) distro, as by design it isolates each service and its configuration from each other, to allow both reproducibility (e.g. configuration rollback) and running multiple instances of a service side-by-side.

Note that I've only fully tested these patches on top of `5.2.0`, which required changing the first patch to:

```diff
diff -Naur a/src/libstrongswan/library.c b/src/libstrongswan/library.c
--- a/src/libstrongswan/library.c       2014-06-05 03:50:30.000000000 -0400
+++ b/src/libstrongswan/library.c       2014-09-30 15:25:27.927757711 -0400
@@ -307,7 +307,7 @@
 #ifdef STRONGSWAN_CONF
        if (!settings)
        {
-               settings = STRONGSWAN_CONF;
+               settings = getenv("STRONGSWAN_CONF") ?: STRONGSWAN_CONF;
        }
 #endif
        this->public.settings = settings_create(settings);
```