lib/time-util: copy relative time parsing functions from systemd

The functions are copied nearly as-is.  Coding style has been modified to
match with util-linux project, while the functionality remains untouched.

CC: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
last: parse easy to use time formats

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