sadf: New option -t added.

sadf: New option -t added.

First, this patch renames sadf option '-T' into '-U', and
sadf option '-t' into '-T'.
It then adds a new option: -t. This option tells sadc to display
the timestamps in the local time of the data file creator
instead of UTC. The same option already exists for sar.
The FAQ is also updated: Tell that options -s and -e are always
expressed in local time.