Set XDG_CURRENT_DESKTOP for the started session

See https://git.reviewboard.kde.org/r/117400 for details
Merge pull request #139 from MartinBriza/desktopsession

Set XDG_CURRENT_DESKTOP for the started session
Fix UserModel.cpp to use getpwent()

Per open bug report #123

Also fix some problems with passwd struct argument
per comments from @aavci1
Merge pull request #141 from Teknocrat/getpwd-support

Fix UserModel.cpp to use getpwent()