ifupdown2: fixup maintainer scripts to not crash on files not existing

ifupdown2: fixup maintainer scripts to not crash on files not existing

Ticket: CM-5735
Reviewed By: no one
Testing Done: lots of repo upgrade testing by John B.

This partially reverts commit 8959e8e4ec93487228733b17ef2b220ed47c1260 ("Fix
bash completion for sudo users (mostly cosmetic)") in the sections where
the Debian maintainer scripts were modified.

In addition fixed up the Debian maintainer scripts to prevent a
potential upgrade failure, discussion follows:

The Cause For the upgrade Failure
=================================

Commit 8959e8e4ec93487228733b17ef2b220ed47c1260 introduced changes to
ifupdown2's package maintainer scripts to override some files provided
by the bridge-utils and udev packages as they conflict with ifupdown2.

The change to ifupdown2's postrm script is currently causing the script
to exit with a failure code if the following files do not exist on the
box:
/etc/udev/rules.d/80-networking.rules
/etc/udev/rules.d/60-bridge-network-interface.rules

The reason for this is in how the script is written.

Since the postrm script is configured to exit on the first command
failure, the "set -e" on line 3, the commands on lines 20 and 22 will
fail if the files being looked for do not exist, causing the script to
exit with a non-zero exit code. Causing apt to fail the upgrade.

.postrm script excerpt
----------
20 udevlink=$(readlink /etc/udev/rules.d/80-networking.rules 2>/dev/null)
21 [ -n "$udevlink" -a "$udevlink" == "/dev/null" ] && rm -f /etc/udev/rules.d/80-networking.rules
22 udevlink=$(readlink /etc/udev/rules.d/60-bridge-network-interface.rules 2>/dev/null)
23 [ -n "$udevlink" -a "$udevlink" == "/dev/null" ] && rm -f /etc/udev/rules.d/60-bridge-network-interface.rules
----------

Solution
--------

The solution requires having the newer version of the ifupdown2 package
provide a preinst script that gets run before the older package's postrm
script is run[1,2]. This gives us the opportunity to quietly fix the
problem. We also fix the postrm script in newer versions of the package
so it cannot bomb out in this way.

Finally, removed what appears to be a poor merge from commit
5f5d84e3261bf70dd3541666a3d3a7f817727ce6.

Fixes: 8959e8e4ec93487228733b17ef2b220ed47c1260
(cherry picked from commit 0eab79eb5aed4cdf0674e3f0ddad3631dd457bdc)