Disallow column mode (DECSET/DECRST 3) by default

GNU Screen sends the following sequence and resets DECCOLM(set to 80 column mode) at the start up time.

> ESC [ ? 3 ; 4 l

With this sequence, current iTerm2 resizes its screen size to 80 columns, since VT100Termnal.allowColumnMode (DECSET/DECRST 40) is YES by default.
This behavior seems to be incompatible with other major PC-terminals (such as xterm). So I propose that we change the default value of allowColumnMode to NO.