PluginSpec platform: More deterministic name for Linux
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 <012f8d714267bccc9d4766fbeace9f175fcc70c4@digia.com>

kuba1/qtcreator,xianian/qt-creator,xianian/qt-creator,farseerri/git_code,maui-packages/qt-creator,farseerri/git_code,farseerri/git_code,martyone/sailfish-qtcreator,xianian/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,maui-packages/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,maui-packages/qt-creator,AltarBeastiful/qt-creator,martyone/sailfish-qtcreator,maui-packages/qt-creator,AltarBeastiful/qt-creator,kuba1/qtcreator,amyvmiwei/qt-creator,amyvmiwei/qt-creator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,danimo/qt-creator,martyone/sailfish-qtcreator,Distrotech/qtcreator,kuba1/qtcreator,maui-packages/qt-creator,AltarBeastiful/qt-creator,amyvmiwei/qt-creator,xianian/qt-creator,amyvmiwei/qt-creator,Distrotech/qtcreator,maui-packages/qt-creator,xianian/qt-creator,martyone/sailfish-qtcreator,danimo/qt-creator,kuba1/qtcreator,Distrotech/qtcreator,danimo/qt-creator,martyone/sailfish-qtcreator,Distrotech/qtcreator,xianian/qt-creator,amyvmiwei/qt-creator,kuba1/qtcreator,Distrotech/qtcreator,danimo/qt-creator,danimo/qt-creator,farseerri/git_code,danimo/qt-creator,farseerri/git_code,danimo/qt-creator,Distrotech/qtcreator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,farseerri/git_code,AltarBeastiful/qt-creator,xianian/qt-creator,danimo/qt-creator,AltarBeastiful/qt-creator,farseerri/git_code,amyvmiwei/qt-creator,kuba1/qtcreator,kuba1/qtcreator,farseerri/git_code,maui-packages/qt-creator,xianian/qt-creator,Distrotech/qtcreator,danimo/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator