last: add --since time spec option

The --since and --until options are companions often needed together.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add --since and time option formats to last(1) manual

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: fix --present option logic error

Printing of sessions 'still logged in' was skipped because an error in
logic.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: use configuration struct

This allows reducing global variables and will minimize number of
arguments for functions making code a little bit easier to read, and
maintain.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: add --time-format with iso-8601 format

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