logs-show: rename "ts" paramater/variable to "display_ts"

logs-show: rename "ts" paramater/variable to "display_ts"

When displaying log data we deal with two kind of timestamps: the one we
use for display (typically the source timestamp if available), and the
one we use internally (typically the reception timestamp of journald).

The user-facing output modes generally use the display timestamp, the
ones intended for further processing (i.e. json + export outputs) do
not, and directly query the timestamps of the entry, ignoring the
source. This gets a bit confusing, since it's not always clear why we
use which timestamp where. Let's address that by renaming the generic
"ts" parameter/variable to "display_ts" to emphasize that the stored
timestamp are "corrected" timestamps for display only.

No real code change, just some renaming.