make copy_file() a bit easier to understand, and smaller

function                                             old     new   delta
copy_file                                           1565    1447    -118
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-118)           Total: -118 bytes
   text    data     bss     dec     hex filename
 770938    1063   10788  782789   bf1c5 busybox_old
 770814    1063   10788  782665   bf149 busybox_unstripped

4 files changed