Make the check for the availability of fork() more precise

[eregon]

So that it also triggers for TruffleRuby which currently doesn't support fork.

Methods can have respond_to? => false if they are "unimplemented"/"not supported on this platform", but yet be actually defined and be part of Module#methods.
Those methods usually just raise a NotImplementedError like
fork is not available (NotImplementedError).
This is what MRI does and it's also supported in at least JRuby, Rubinius and TruffleRuby.

The error message is made generic and now looks like this:

# For TruffleRuby
ERROR: worker mode not supported on truffleruby on this platform
# For JRuby
ERROR: worker mode not supported on jruby on this platform

Related: I wonder, why does the cluster mode of Puma needs fork and not just spawn?

[eregon]

The CI fails, but it seems unrelated to this change and it already fails on master.

[eregon]

Could someone review this PR?
cc @evanphx @schneems @luislavena @nateberkopec

[schneems]

Thanks!