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.

unix/x11misc.h

@@ -0,0 +1,15 @@
+/*
+ * x11misc.h: header file for functions that need to refer to Xlib
+ * data types. Has to be separate from unix.h so that we can include
+ * it only after including the X headers, which in turn has to be done
+ * after putty.h has told us whether NOT_X_WINDOWS is defined.
+ */
+
+#ifndef NOT_X_WINDOWS
+
+/*
+ * x11misc.c.
+ */
+void x11_ignore_error(Display *disp, unsigned char errcode);
+
+#endif