Modified F-key logic to simulate Mintty

In what Putty calls "normal" mode, it sends odd function keys,
inconsistent with urxvt, xterm, or Mintty. In "Xterm R6" mode, the keys
initially seem consistent with the other terminals, but further testing
revealed that meta keys get ignored in this mode. I have modified this
mode to transmit the same sequences as Mintty (note: these vary slightly
from urxvt and I picked these to create a stable fully-functional Mintty
commit, in future commit I will change these to match urxvt instead).