kill: missing RTMAX/RTMIN on Linux

[BenWiederhake]

(Originally documented in #6202)

On Linux, GNU 9.5 kill on Linux lists many RTMIN/RTMAX-related signals. procps kill does NOT list these signals.

I'm not entirely sure what they mean, so perhaps we simply don't want to support it, but I wanted to mention it, since it is not mentioned in any other issue.

[ChrisDryden]

This is also impacting out env-signal-handler gnu test. It appears at some point early on in development the use of timeout and yes in the test was swapped out with GNU and when I revert that change those tests go back to fail

[ChrisDryden]

The implementation for this will be messy since Nix does not handle RTMIN and RTMAX so it will inevitable require some unsafe calls.