Add support for set comments

[turekt]

Per issue #288, I've added a simple change to sets to support adding comments.

[stapelberg]

Thanks for the PR!

But, your new test seems to fail, at least on GitHub Actions:

--- FAIL: TestSetComment (0.00s)
    nftables_test.go:5954: message 3: got -- want
          01 00 00 00 -- 01 00 00 00
          0b 00 01 00 -- 0b 00 01 00
          66 69 6c 74 -- 66 69 6c 74
          65 72 00 00 -- 65 72 00 00
          0c 00 02 00 -- 0c 00 02 00
          73 65 74 6e -- 73 65 74 6e
          61 6d 65 00 -- 61 6d 65 00
          08 00 03 00 -- 08 00 03 00
          00 00 00 00 -- 00 00 00 00
          08 00 04 00 -- 08 00 04 00
          00 00 00 07 -- 00 00 00 07
          08 00 05 00 -- 08 00 05 00
          00 00 00 04 -- 00 00 00 04
          08 00 0a 00 -- 08 00 0a 00
        ! 00 00 00 02 -- 00 00 00 12
          13 00 0d 00 -- 13 00 0d 00
          07 0d 74 65 -- 07 0d 74 65
          73 74 20 63 -- 73 74 20 63
          6f 6d 6d 65 -- 6f 6d 6d 65
          6e 74 00 00 -- 6e 74 00 00
           -- 
FAIL

Can you take a look please?

[turekt]

Set ID may vary per system so its value is now fixed. Should be resolved.