Added -T <tty> and -n (uninterruptible) options

I've also merged code from other forks and added a man page, a more distro-friendly Makefile and a few code cleanups.

-T takes a single argument <tty> which must be a valid accessible character device (e.g., /dev/tty9, /dev/pts/1) and -n prevents the program from quitting using keyboard commands. (signal process to quit: pkill -INT tty-clock)

These 2 options allow for better integration with the inittab(5) file on Linux, tty-clock could be set to run permanently on one of the virtual consoles for as long as the system is up or only under certain init levels.