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.

tests/unittests/log/stub_log.c

@@ -49,6 +49,10 @@ log_level_t log_level_from_string(char *log_level)
     return (log_level_t)mock();
 }
 
+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) {}