Ignore X11 BadMatch errors during cut buffer setup.

Ignore X11 BadMatch errors during cut buffer setup.

This is quite a pain, since it involves inventing an entire new piece
of infrastructure to install a custom Xlib error handler and give it a
queue of things to do. But it fixes a bug in which Unix pterm/PuTTY
crash out at startup if one of the root window's CUT_BUFFERn
properties contains something of a type other than STRING - in
particular, UTF8_STRING is not unheard-of.

For example, run
  xprop -root -format CUT_BUFFER3 8u -set CUT_BUFFER3 "thingy"
and then pterm without this fix would have crashed.