lua/log: declare say_logger_init and say_logger_initialized

We gonna use it to provide a way to initialize
logger subsystem early.

Part-of #689

Reviewed-by: Oleg Babin <olegrok@tarantool.org>
Reviewed-by: Leonid Vasiliev <lvasiliev@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/lua/log.lua

@@ -22,6 +22,12 @@ ffi.cdef[[
     void
     say_set_log_format(enum say_format format);
 
+    extern void
+    say_logger_init(const char *init_str, int level, int nonblock,
+                    const char *format, int background);
+
+    extern bool
+    say_logger_initialized(void);
 
     extern sayfunc_t _say;
     extern struct ev_loop;