Replace isc_log_create/destroy with isc_logconfig_get()

Add isc_logconfig_get() function to get the current logconfig and use
the getter to replace most of the little dancing around setting up
logging in the tools. Thus:

    isc_log_create(mctx, &lctx, &logconfig);
    isc_log_setcontext(lctx);
    dns_log_setcontext(lctx);
    ...
    ...use lcfg...
    ...
    isc_log_destroy();

is now only:

    logconfig = isc_logconfig_get(lctx);
    ...use lcfg...

For thread-safety, isc_logconfig_get() should be surrounded by RCU read
lock, but since we never use isc_logconfig_get() in threaded context,
the only place where it is actually used (but not really needed) is
named_log_init().

lib/ns/include/ns/log.h

@@ -60,13 +60,3 @@ ns_log_init(isc_log_t *lctx);
  *\li	The categories and modules defined above are available for
  * 	use by isc_log_usechannnel() and isc_log_write().
  */
-
-void
-ns_log_setcontext(isc_log_t *lctx);
-/*%<
- * Make the libns library use the provided context for logging internal
- * messages.
- *
- * Requires:
- *\li	lctx is a valid logging context.
- */

lib/ns/log.c

@@ -50,13 +50,6 @@ isc_log_t *ns_lctx = NULL;
 
 void
 ns_log_init(isc_log_t *lctx) {
-	REQUIRE(lctx != NULL);
-
 	isc_log_registercategories(lctx, ns_categories);
 	isc_log_registermodules(lctx, ns_modules);
 }
-
-void
-ns_log_setcontext(isc_log_t *lctx) {
-	ns_lctx = lctx;
-}