OpenBSD support

[umeldt]

Current main doesn't build at all on OpenBSD, but including the same headers as for FreeBSD in src/ipaddr/extension.c takes care of that.

After building, most of the tests seem to run fine, but the following from the time suite fail:

31_01|0
31_02|0
31_03|0
32_01|0
32_02|0
32_03|0
34_03|0
34_04|0
34_07|0
34_08|0
34_11|0
34_12|0

After taking a quick look it seems to be related to how time comparisons are handled - I'd be happy to dig into this further if I find the time...!

time_before behavior on MacOS:

sqlite> .load dist/time
sqlite> select time_before(time_date(2011, 11, 18), time_date(2011, 11, 19));
1

...and on OpenBSD:

sqlite> .load dist/time
sqlite> select time_before(time_date(2011, 11, 18), time_date(2011, 11, 19));
-955290111