Do not read stderr from proxy autodetection scripts

Do not read stderr from proxy autodetection scripts

This fixes a regression introduced in
  commit 8f858d560e3b7b475c623c4e242d1edce246025a

  don't leak FD in AutoProxyDetect command return parsing

which accidentally made the proxy autodetection code also read
the scripts output on stderr, not only on stdout when it switched
the code from popen() to Popen().

Reported-By: Tim Small <tim@seoss.co.uk>