security: avoid swapping sensitive data in linux

in order to harden the management of the master passphrase, we could try to avoid swapping the memory areas that contain it.

http://stackoverflow.com/questions/29524020/prevent-ram-from-paging-to-swap-area-mlock https://groups.google.com/forum/#!topic/comp.lang.python/2vaRic8xPIs

for reference, gnome-keyring tries to avoid swapping.

(from redmine: created on 2016-05-17, relates #8107 (closed))