log: add common helper log_set_target_and_open()

quite often we want to set a log target and immediately open it. Add a
common helper for that.

src/basic/log.h

@@ -51,6 +51,7 @@ static inline void clear_log_syntax_callback(dummy_t *dummy) {
 const char *log_target_to_string(LogTarget target) _const_;
 LogTarget log_target_from_string(const char *s) _pure_;
 void log_set_target(LogTarget target);
+void log_set_target_and_open(LogTarget target);
 int log_set_target_from_string(const char *e);
 LogTarget log_get_target(void) _pure_;