[lipstick] serialize/hide devicelock plugin arguments

[rainisto]

Do not merge untill devicelock encryption plugin pull request have been merged.
This pull request serializes commandline arguments so it can be piped into encryption plugin's stdin.

EDIT: new devicelock plugin has now been merged to devel. So its when this receives LGTM then someone with merge rights can merge this in.

[pvuorela]

Inconsistent indentation.

[giucam]

This should be &.

[mikkoharju]

This will probably call QByteArray::append(char ch)?

[rainisto]

Rebased and updated against review comments. And verified functionality against latest version of devicelock plugin in devel.

[special]

Document (or fix) that limit of 127 characters per argument. Also add a check for it here, or the overflow behavior is a potential problem.

[thp]

Should this be merged or dropped? @rainisto @special

[rainisto]

Well it depends, merged if we want to fix/hide plugin arguments from current plugin and dropped if plugin rewrite will use different approach. I haven't seen any plans how the new/rewritten plugin will work so I don't know if it will want to reuse this approach or not. Personally I would vote for merge as it improves security on current device, but if rewrite takes different approach, then I would keep this open until @special or someone doing the rewritten plugin comments how they plan to pass the parameters to the plugin.