[windows] Windows command for file size only

Use a function to get rid off some limitation in the ~z operator. It is especially useful with a for loop:

@echo off
set size=0
call :filesize "C:\backup\20120714-0035\error.log"
echo file size is %size%
goto :eof

:: Set filesize of first argument in %size% variable, and return
:filesize
  set size=%~z1
  exit /b 0