Support relative expiry times in signurl.

UNIX timestamps are annoying on the command line, eg. compare:

 $ s3cmd signurl s3://bucket/filename $(($(date +%s) + 60))

vs.

 $ s3cmd signurl s3://bucket/filename +60

Both times are still relative to the local clock of course, but at least
this is easier to read.

Signed-off-by: Chris Lamb <chris@chris-lamb.co.uk>