V1.1 Add a capability to show n past messages or from last boot.

Added a capability to showing past messages. Limited by journal_history_max and from_boot_enabled:

• journal_history_max by itself could be used to start journo from an arbitrary number of entries into the past.
• from_boot_enabled will begin the display of past entries from the last boot time, but these may be pruned by journal_history_max.

This feature is only intended for going back a reasonable number of entries, perhaps a days worth or so on a desktop. There is no constraint on RAM usage, so it would be unwise to go back to the last boot of a machine that has been up 400 days. In such cases I would set from_boot_enabled to no and rely on a sensible number for journal_history_max. Note journal_history_max defaults to 500.