拾取一点和拾取另一点时, 实现线段跟随的效果, 实现右键，空格，ESC都可以退出效果

/// <summary>
/// 交互得到另一点 // houlinbo QQ420021327
/// </summary>
/// <returns></returns>
public static PromptPointResult SelectOtherPt(string strMsg, Point3d pt)
{
Document doc = Application.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
PromptPointOptions prpointoptions = new PromptPointOptions(strMsg);
prpointoptions.BasePoint = pt; //
prpointoptions.UseBasePoint = true; //可实现线段跟随的效果
prpointoptions.AllowNone = true; //右键，空格，ESC都可以退出
PromptPointResult prPointRes = ed.GetPoint(prpointoptions);
return prPointRes;
}