Fixes for dcast.data.table

+ Workaround for segfault in dcast.data.table due to existing bug #5191.
+ Error in dcast.data.table if input DT has duplicated names
+ Error if data to cast is empty
+ Added more tests to dcast.data.table
+ Minor fixes in ?dcast.data.table

+ Fixed a broken link in ?setorder. ?forder doesn't exist yet.