Resolve hosts by DNS name in separate threads so we can cancel them

getaddrinfo(3) may block a long time so proper termination of the daemon may
block if DNS servers are not reachable.

getaddrinfo(3) is an optional cancellation point in posix threads so it
might still block a shutdown but at least on Android (with the signal based
pthread_cancel implementation) it works, on Linux starter will kill charon
anyway after a while.

src/libstrongswan/library.h

@@ -77,6 +77,7 @@
 #include "printf_hook.h"
 #include "utils.h"
 #include "chunk.h"
+#include "host_resolver.h"
 #include "settings.h"
 #include "integrity_checker.h"
 #include "processing/processor.h"
@@ -170,6 +171,11 @@ struct library_t {
 	 */
 	scheduler_t *scheduler;
 
+	/**
+	 * resolve hosts by DNS name
+	 */
+	host_resolver_t *hosts;
+
 	/**
 	 * various settings loaded from settings file
 	 */