PluginSpec platform: More deterministic name for Linux

There is not even a guarantee that VERSION from os-release contains
"Linux" anywhere. Prepend "Linux" or "Unix" and add the version details
in parentheses instead.
Also remove unnecessary (and wrong) /etc/issues fallback.

Change-Id: I0457ad9e8cedeea35e0cad5ea8093652eb8428e1
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Plugin manager: Show current platform string in plugin details

Change-Id: Ia1fc03ae715daa9f0469f6b2859de932f8eeb562
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>