Add option to set minimum value of accepted work

If the available work is not above some minimum value, put the miner to
sleep.
Also, changed the total value display to use the already calculated
value in the pubkeybatch_t.
Update notice about work not meeting min value

To avoid confusion, set the was_sleeping flag to 1 when value of work
isn't high enough, so that it doesn't display "No work is available,
sleeping."
Test for pkb before accessing pkb->total_value
Document -P option
Merge remote-tracking branch 'fizzisist/master'