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.c

@@ -27,6 +27,7 @@
 #include <selectors/traffic_selector.h>
 
 #define CHECKSUM_LIBRARY IPSEC_LIB_DIR"/libchecksum.so"
+#define HOST_RESOLVER_MAX_THREADS 2
 
 typedef struct private_library_t private_library_t;
 
@@ -68,6 +69,7 @@ void library_deinit()
 	this->public.scheduler->destroy(this->public.scheduler);
 	this->public.processor->destroy(this->public.processor);
 	this->public.plugins->destroy(this->public.plugins);
+	this->public.hosts->destroy(this->public.hosts);
 	this->public.settings->destroy(this->public.settings);
 	this->public.credmgr->destroy(this->public.credmgr);
 	this->public.creds->destroy(this->public.creds);
@@ -182,6 +184,7 @@ bool library_init(char *settings)
 
 	this->objects = hashtable_create((hashtable_hash_t)hash,
 									 (hashtable_equals_t)equals, 4);
+	this->public.hosts = host_resolver_create(HOST_RESOLVER_MAX_THREADS);
 	this->public.settings = settings_create(settings);
 	this->public.proposal = proposal_keywords_create();
 	this->public.crypto = crypto_factory_create();