Improve landscape-config handling of client.conf changes

[Jerry Seutter]

The way dpkg-reconfigure is handled has a some minor issues:

- part of client.conf is written by landscape-client.postinst, the rest is written by landscape-config. If the second part fails, client.conf is partially written and settings are lost.
- Creating backups of client.conf leads to error messages about non-empty directories when the package is uninstalled.

In general the following should hold:
- landscape-config should be responsible for modifying client.conf
- landscape-config gathers settings from the existing config file as well as the command line, with command line settings taking precedence.
- Comments in client.conf should be preserved.
- No unnecessary rewrites of client.conf. If nothing is different, do not rewrite the file.
- No backups of client.conf

The issue of preserving comments is not possible with ConfigParse as it currently exists.
The issue of no unnecessary rewrites is documented in bug #940480.