utlist: add CDL_CONCAT

[xunicatt]

Add missing CDL_CONCAT macro for circular doubly-linked lists to match LL_CONCAT and DL_CONCAT.
This fills the API gap noted in issue #279.

[Quuxplusone]

Ack — will take a look sometime soon.
I recently revamped the PREPEND and PREPEND_ELEM test (test86) to be "chummier with the implementation." I think I'd like to see the CONCAT tests (test63 and test64) combined with your new test to use that same style. I'm not sure if that's a good use of your time, though, or if I should just find time to do it myself.

[xunicatt]

Ack — will take a look sometime soon. I recently revamped the PREPEND and PREPEND_ELEM test (test86) to be "chummier with the implementation." I think I'd like to see the CONCAT tests (test63 and test64) combined with your new test to use that same style. I'm not sure if that's a good use of your time, though, or if I should just find time to do it myself.

Yup I can implement a test like that.

[xunicatt]

Hey @Quuxplusone, would you mind taking a look at this when you have time?

[Quuxplusone]

Merged 5ada598 — thanks!
(I rewrote the unit test in the style I was aiming for. The "utlist.h" change remains the same as what you had, which LGTM.)