ifupdown: don't use PPP updetach option by default

I am being bitten by the *exact* set of circumstances as in Debian bug #675016

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=675016

In my case, it brought my network to a complete standstill, until I had someone physically go and visit the machine.

This appears to have been fixed in later versions of the ifup package and *only* affects 12.04 LTS

1.

lsb_release -rd
Description: Ubuntu 12.04.1 LTS
Release: 12.04

2.

# apt-cache policy ifupdown
ifupdown:
  Installed: 0.7~beta2ubuntu8
  Candidate: 0.7~beta2ubuntu8
  Version table:
 *** 0.7~beta2ubuntu8 0
        500 http://archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status