Return valid exit code from Windows batch scripts.

Technique used for setting Windows batch script exit codes in Boost.Build worked
correctly when the script is called from a Windows shell process and then checked
from inside that process. However, when run from a temporary shell process that
needs to terminate after running the script, such a process would always return
exit code 0.

This prevented anyone automating those scripts from detecting their success/
failure status by using their exit code without adding an additional batch script
layer.

For example, consider the following two script files:

  ret666.cmd:
    exit /b 666

  wrapper.cmd:
    call ret666.cmd

They both 'should return the value 666' and when run directly from a cmd.exe
console indeed both do (they set the shell process's ERRORLEVEL environment
variable to 666). However, when run like this:
  cmd /c <script-name>
running ret666.cmd causes the temporary cmd.exe process to exit with exit code
666, while running wrapper.cmd causes it to exit with exit code 0.