Block SIGCHLD during select.

Select was losing child termination signals so we explictly
block SIGCHLD during select, and unblock after select returns.
Tested on 256 core smp system, no hangs or zombies.  Thanks
to Alain Miniussi <alain.miniussi@oca.eu> for reporting this.
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.