Implement RpcCancelThreadEx using existing code moved into a new
function, rpc_ccancel_thread.

Implement RpcCancelThread based on RpcCancelThreadEx with timeout of 0.

This also reverts commit 2b52dd84:
wordpad: Empty the richedit undo buffer on creation.

The reverted commit I created to fix an issue that only applied to Wine,
but it just masked the issue which was in richedit controls.  The
default character format was set in two places while wordpad was
starting up, and caused wordpad to have two undo items at startup.

Trying to set the font size to a value larger than 1638
in points (yHeightCharPtsMost) using EM_SETCHARFORMAT will cause it to be
set to actually set to the maximum.