Print current unixtime

Summary:
This command tweak was first implemented on UnderNet #win
in 2000s.

It's useful to write data in database or config files
when such timestamp format is used.

To avoid any confusion in variable names, the difference
between current and input timestamps is renamed delta_t.

Test Plan:
Check unixtime without parameter prints current timestamp.

Check an unixtime as parameter still works.

Check 0 behaves as a regular input, not as a falsy value.

21:09:16 < Dereckson> Gizmo: unixtime
21:09:16 < Gizmo> Dereckson: 1486674556
21:09:20 < Dereckson> Gizmo: unixtime 1486673868
21:09:20 < Gizmo> Dereckson: -11 minutes, -32 secs.
21:09:32 < Dereckson> Gizmo: unixtime 0
21:09:32 < Gizmo> Dereckson: -24777909 minutes, -32 secs.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D914