kdepasswm kcm: Fall back to the system full name when necessary.

When loading the KCM data and KEmailSettings::RealName is empty, fall
back to using the user's real name from /etc/passwd or equivalent via
KUser.

To quote the original review request: "Besides, such method seems
natural, considering that change of real username in kcm module calls
chfn to change real name in finger db, which will be reflected back in
KUser::FullName."

REVIEW: 104965
Submitted by: Aleksey Yermakov