RCP: X.X, improve TXM update management under Linux

Currently the TXM update mecanism relies on a specific access rights policy on TXM files to work.

In some cases this policy can not work.

We should change the update mecanism to be more compatible with usual access rights policies.

Solution 1.a

Raise user privileges for TXM update operations with gksu or PolicyKit

Solution 1.b

Use a ‘setuid’ strategy (start with root privileges by suid bit, lower privileges for usual sessions by setuid and raise privileges only for TXM update operations) or any privilege mecanism available for a Linux application to udpate with or without the system software update mecanism.

Solution 2

Change TXM components architecture to allow any user or only the current user or only the system software update mecanism to update TXM.

(from redmine: issue id 2318, created on 2018/01/11 by Serge Heiden)