Fix compilation of the ports event multiplexer

[rgacogne]

Short description

I tested that this fixes the compilation and works on OpenIndiana.
Also fixes the compilation of our dns_random unit tests on systems that have both getrandomm() and arc4random().

Fixes #9025.

Note that even though we do compile the Solaris completion ports code, we always also compile the /dev/poll one, and we do prefer that second one since it's loaded first and they have the same priority. I think we should just remove the Solaris completion ports code in later releases.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)