last: never restrict session time

When a session time will reach whopping 10000 days the last round bracket
is unnecessarily removed from output.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: add last(1) test

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: trim trailing white spaces

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: tell verbally system is still running

Use of uptime time stamp as previous boot login time makes the output not
constant, which is rather difficult to test.  Verbal message 'system is
still running' makes testing easy, and noticing which boot is still
running clear to a person.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>