Add PC-style function keys and arrow keys (including modifiers) for Xter...

With these modifications to Xterm R6 I am able to use function keys and arrow keys combined with SHIFT-CTRL-ALT when connecting to byobu/tmux on a remote machine running Ubuntu.

tmux is using the PC-style function keys according to this documentation:
 http://invisible-island.net/xterm/ctlseqs/ctlseqs.html

I have found it extremely hard to figure out what the standard xterm really means and during what circumstances the PC-style function keys apply. If the document above describes what PuTTY means to implement as "XTerm R6", then my changes should be considered more correct than the current implementation.