work-week ends do not seem to be calculated correctly

Work week endings may be calculated incorrectly. Observe the following:
```
$ for ww in {s,e}o{,p,n}ww
do printf "%s\t%s\n" $ww $(
faketime '2017-03-05 12:34:56' task calc $ww
); done

soww 2017-03-13T00:00:00
sopww 2017-02-27T00:00:00
sonww 2017-03-13T00:00:00
eoww 2017-03-10T23:59:59
eopww 2017-03-03T23:59:59
eonww 2017-03-17T23:59:59
```
with the reference date of 2017-03-05 12:34:56 (a Sunday), and the calendar:
```
$ ncal 03 2017
March 2017
Mo 6 13 20 27
Tu 7 14 21 28
We 1 8 15 22 29
Th 2 9 16 23 30
Fr 3 10 17 24 31
Sa 4 11 18 25
Su 5 12 19 26
9 10 11 12 13
```
these do not look right:
- It seems to consider the next "work week" to have already begun on Saturday, but I should think the next work week doesn't actually begin until Monday. Even though there's no actual work days left, it's still within the same work week.
- `soww` and `sonww` are identical
- `eoww` is earlier than `soww` (!)

This issue was noticed during testing for #47 (a change to comments documenting behavior of `eo*`) and is forked into a standalone issue as per the discussion there.

(test version of `libshared` was 591fa58)