More extraction of functionality

Extracting server and tty functions into separate headers.
Thread functions for new connections, tty data and client data
are also extracted and used as separate functional blocks.

common.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <time.h>
+#include <fcntl.h>
+
+#define DATABUF_LEN 128
+
+#define TIMESTAMP_FORMAT "%Y-%m-%dT%H:%M:%S" /* ISO 8601 */
+#define TIMESTAMP_LEN 20+1 /* calculated according to the timestamp format */
+
+/* Global variables used throughout the application. */
+int debug_messages;		/* if > 0 debug messages will be printed */
+
+/* Global functions used throughout the application. */
+
+/**
+ * Converts time in seconds from Epoch to a conveniently formatted string.
+ */
+void time2string(time_t time, char* timestamp);