trace: accept time units in --since and --until

The timestamp typically will come from 'trace.py list' output.  This
change makes it easier to paste the timestamp. Listing prints the time
with '.' separating seconds from nanoseconds.  The options '--since'
and '--until' accept timestamp in nanoseconds so the dot must have
been removed. Now one only needs to append the 's' suffix which
changes the unit to seconds.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>