Drop privileges when reading the salt file

Summary:
As found by Matthias Gerstner the user here controls nearly everything:

  - he controls his own password
  - he controls where the salt is read from
  - he can read the final salted hash (e.g. by calling strace() on kwalletd at
    the right time)

By using this fact he can do the following things:

  - test for existence of files in locations otherwise not accessible
  - exploit an information leak. 56 bytes of root owned files will be provided
    to him in the form of a salted hash. He won't be able to easily retrieve
    the original "salt" again. But if the "salt" comes from a well structured
    input file then the possible input combinations can suddenly be quite
    limited and a brute force attack can be feasible to gain knowledge of
    certain root-owned data.
  - the fact that the user can cause a root-owned process to read 56 bytes
    from an arbitrary file in the system could have other side effects
    depending on the situation in the system. E.g. FUSE, pseudo file systems
    or device files might react specially to this.

This is a very theoretical attack, but since it's reasonable easy to fix it, let's do it :)

Test Plan: kwallet-pam still works

Reviewers: dakon

Reviewed By: dakon

Subscribers: dakon, mgerstner, fvogt, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12937