log: Introduced stderr log handler

This handler redirects stderr output to a pipe and gathers logs from the
read end.

The handler is called from main loop, therefore, if a function prints logs
to stderr they will be put to log file only after function returns.

src/log.h

@@ -63,6 +63,10 @@ void log_msg(log_level_t level, const char * const area,
     const char * const msg);
 log_level_t log_level_from_string(char *log_level);
 
+void log_stderr_init(log_level_t level);
+void log_stderr_close(void);
+void log_stderr_handler(void);
+
 void chat_log_init(void);
 
 void chat_log_msg_out(const char * const barejid, const char * const msg);