trace: use seconds as the default unit of timestamp

trace.py allows to slice samples to a time range using --since,
--until and --period. Currently the default unit for timestamps was
nanoseconds. However the timestamp pretty much always comes from the
'trace list' output, which displays timestamp in seconds with a dot
separating the nanosecond precision part. That required explicit unit
designating by appending 's' suffix to the timestamp when passing to
--since and --until to

This change makes --since and --until use seconds as a default unit,
so that the timestamp copied from trace list can be used without any
change, eg:

 $ scripts/trace.py --since 1400607382.775557041

Default unit of --period (nanoseconds) remains unchanged.

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