Bug 16485

# Description of problem:

Online (CLI) upgrade from Mageia 4 to Mageia 5
over-wrote working /etc/exports file with a file empty of definitions.
This machine is mostly used as a headless server, as a dual-core 64-bit 'muscle car', to my other motley collection of old machines, so NFS exports is central to application.
Luckily I also backup system /etc configs as well as user /home data.

# Version-Release number of selected component (if applicable):

nfs-utils-1.3.0-6.mga5.src.rpm

# How reproducible:

Only need to upgrade Version 4 to 5 once, thus only appeared (a critical) once.
 'rpm -qif /etc/exports' reports 'name: nfs-utils ........'
So blank file is hard-coded into upgrade rpm, and will overwrite every time?
I shall be doing another upgrade tomorrow on a different installation
of Mageia 5 to Caldron(6) to see if same happens.

# Steps to Reproduce:
1.Follow instructions near end of:
  https://wiki.mageia.org/en/Mageia_5_Release_Notes
  "Upgrading online, using urpmi (CLI)" 

2. su
   urpmi.removemedia -a
   urpmi.addmedia --distrib --mirrorlist 'http://mirrors.mageia.org/api/mageia.5.x86_64.list'
   urpmi --replacefiles --auto-update --auto --download-all --test
   urpmi --replacefiles --auto-update --auto
 
(the 'urpmi ... --download-all --test' line unnecessary to reproduce bug but I did it anyway to ensure some safety) 


3. Then reboot:
     shutdown -r now

   Amazing! Everything comes back as before;
   including samba 'exports' tested via a Windows 7 netbook.
   So trouble free; but is it really running the new version?
      echo $MACHTYPE
        x86_64-mageia-linux-gnu
      ls -l /boot
        ....vmlinuz -> vmlinuz-3.19.8-desktop-3.mga5
   So as far as simple checks it indeed is version 5
  (Congratulations to team on their work!!)

4. However:
   Moving to another machine (a VIA C3 running Mageia 2, or VectorLinux 6)
   which previous to server upgrade was perfectly OK,
   with four mounted exports from server (plus a few more from a Raspberry Pi).
   Attempting:
     sudo mount [servername]:/[exported-tree] /[local-mount]
   -or using predefined /etc/fstab mount points:
     sudo mount -a
   -get response:
     'servername:/export-tree Permission denied'

# My fix, replace blank upgrade with working backup, and make NFSd reload it:
  su
  cp -vf [my-sys-backup]/etc/exports /etc/exports
  exportfs

# Suggestions for proper fix:
  Remove /etc/exports from package so it cannot overwrite a good existing file.
  -Does NFSd (or exportfs) crash if /etc/exports not present?

  Or:
  If /etc/exports exists, backup somewhere, then restore at end of upgrade.
  -This should also be compatible with fresh installs to formatted partition. 

  Or simply:
  Warn users to backup /etc system files AS WELL AS /home data
  Then give tips for restoration of some files.

  Or:
  What would be really nice would be to integrate upgrade/allow-retained-configs
  into a MCC sub-script module.



Reproducible: 

Steps to Reproduce: