Add $conky_user_time (sf.net #2830919)

Add $conky_user_time (sf.net #2830919)

The bug reporter asks if it is possible to add a variable giving the "current
user time" only, since the variable user_times reports the times for ALL
logged users.
AFAIK, the only info one can gather inside conky, is the login time for the
tty connected to conky's standard input.
This commit adds support for it (it should work on any posix compliant *nix).
Note that in coherence with the definition, the variable is called
conky_user_time (for a single user stand-alone machine used as a desktop
this would be the "current" user time).