clock offset: print negative value in metadata

In the upcoming 2.8, we can print the negative value of clock offset in
the metadata, and expect users to update their viewers to correct any
bug that could prevent them from handling those negative values.

This happens only in cases where the system sets CLOCK_REALTIME to 0
after boot, thus leading to a negative clock offset value between
realtime and monotonic clocks.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>