journalctl: add --list-boots to show boot IDs and times

Suggested by David Wilkins <dwilkins@maths.tcd.ie> in
https://bugzilla.redhat.com/show_bug.cgi?id=967521:

> [Specific boot ID is a] bit of a palaver to obtain.  I consulted the
> verbose dump of the journal to discover the _BOOT_ID for the
> timestamp, and then generated the journal dump for that boot using
> journalctl _BOOT_ID=foo -o short-monotonic.