Refactor TLSDNS module to work with libuv/ssl directly

* Following the example set in 634bdfb16d, the tlsdns netmgr
  module now uses libuv and SSL primitives directly, rather than
  opening a TLS socket which opens a TCP socket, as the previous
  model was difficult to debug.  Closes #2335.

* Remove the netmgr tls layer (we will have to re-add it for DoH)

* Add isc_tls API to wrap the OpenSSL SSL_CTX object into libisc
  library; move the OpenSSL initialization/deinitialization from dstapi
  needed for OpenSSL 1.0.x to the isc_tls_{initialize,destroy}()

* Add couple of new shims needed for OpenSSL 1.0.x

* When LibreSSL is used, require at least version 2.7.0 that
  has the best OpenSSL 1.1.x compatibility and auto init/deinit

* Enforce OpenSSL 1.1.x usage on Windows

* Added a TLSDNS unit test and implemented a simple TLSDNS echo
  server and client.

lib/ns/tests/nstest.c

@@ -78,7 +78,7 @@ atomic_uint_fast32_t client_refs[32];
 atomic_uintptr_t client_addrs[32];
 
 void
-isc_nmhandle_attach(isc_nmhandle_t *source, isc_nmhandle_t **targetp) {
+isc__nmhandle_attach(isc_nmhandle_t *source, isc_nmhandle_t **targetp FLARG) {
 	ns_client_t *client = (ns_client_t *)source;
 	int i;
 
@@ -97,7 +97,7 @@ isc_nmhandle_attach(isc_nmhandle_t *source, isc_nmhandle_t **targetp) {
 }
 
 void
-isc_nmhandle_detach(isc_nmhandle_t **handlep) {
+isc__nmhandle_detach(isc_nmhandle_t **handlep FLARG) {
 	isc_nmhandle_t *handle = *handlep;
 	ns_client_t *client = (ns_client_t *)handle;
 	int i;

lib/ns/tests/nstest.h

@@ -62,6 +62,15 @@ extern bool app_running;
 extern int ncpus;
 extern bool debug_mem_record;
 
+#ifdef NETMGR_TRACE
+#define FLARG                                              \
+	, const char *file __attribute__((unused)),        \
+		unsigned int line __attribute__((unused)), \
+		const char *func __attribute__((unused))
+#else
+#define FLARG
+#endif
+
 isc_result_t
 ns_test_begin(FILE *logfile, bool create_managers);