Add support for xterm's 1006 mouse extension

The extension committed in bko 285984 (invented by urxvt's author)
allows the terminal to report coordinates beyond 223. However, for
multiple reasons, xterm's author decided to come up with another
extnsion, addressing the same issue and more.

See more info on this bko 304686

An followup to bko 285984, b876f2a3
Patch by Egmont Koblinger egmont@gmail.com

FEATURE: 304686
FIXED-IN: 4.10