context info: add default_loglevel

Allow this kind of pattern for context creation info preparation

        struct lws_context_creation_info info;

        lws_context_info_defaults(&info, policy);
        info.default_loglevel = LLL_USER | LLL_ERR | LLL_WARN;
        lws_cmdline_option_handle_builtin(argc, argv, &info);

Calling lws_context_info_defaults() zeros down and prepares boilerplate in
the info struct, setting the default_loglevel asserts your application
default for the loglevel and then lws_cmdline_option_handle_builtin() will
set either that default loglevel, or override it from the commandline with
-d 1039 etc

include/libwebsockets/lws-context-vhost.h

@@ -952,6 +952,13 @@ struct lws_context_creation_info {
 	 */
 #endif
 
+	int default_loglevel;
+	/**< CONTEXT: 0 for LLL_USER, LLL_ERR, LLL_WARN, LLL_NOTICE enabled by default when
+	 * using lws_cmdline_option_handle_builtin(), else set to the LLL_ flags you want
+	 * to be the default before calling lws_cmdline_option_handle_builtin().  Your
+	 * selected default loglevel can then be cleanly overridden using -d 1039 etc
+	 * commandline switch */
+
 	/* Add new things just above here ---^
 	 * This is part of the ABI, don't needlessly break compatibility
 	 *

lib/core/libwebsockets.c

@@ -1632,7 +1632,8 @@ lws_cmdline_option_handle_builtin(int argc, const char **argv,
 				  struct lws_context_creation_info *info)
 {
 	const char *p;
-	int n, m, logs = LLL_USER | LLL_ERR | LLL_WARN | LLL_NOTICE;
+	int n, m, logs = info->default_loglevel ? info->default_loglevel :
+				LLL_USER | LLL_ERR | LLL_WARN | LLL_NOTICE;
 #if defined(LWS_WITH_SYS_FAULT_INJECTION)
 	uint64_t seed = (uint64_t)lws_now_usecs();
 #endif