Better implementation of "device changed since time XYZ".

Instead of getting newest change, and seeing if it's newer than
XYZ (which fails if the newest change is a restart), augment
get_action_count to take a time limit, and use this for time-since
queries. More direct and works across restart.
Make "changes today" actually mean something, instead of counting changes
ever recorded, which is what this has long done.

Also, the easy thing to implement is "changes in last day" (meaning
24 hours, or 86400 seconds), not changes since the start of today;
I don't know what's more useful, but make the label explicitly correct.
I felt like the last change was a copout, so I went ahead and implemented
"changes today" as well. Some real usage will tell which (if either) of
these is actually useful.