copyfile有三个参数,前面两个是 源文件和目标文件名, 第三个是表示当目标文件存在时,
是否覆盖,True不覆盖,False为覆盖,Msdn这么说的:
bFailIfExists
Specifies how this operation is to proceed if a file of the same name as that
;specified by lpNewFileName already exists. If this parameter is TRUE and the
new file already exists, the function fails. If this parameter is FALSE and
;the new file already exists, the function overwrites the existing file and
succeeds.