Try to open stdio in non-blocking mode

[waj]

Currently if the stdio descriptors are not a TTY they are re-opened with blocking: true. That makes sense for files, for example if the output is being redirected (i.e: ./foo > somefile). But it doesn't if the input is a pipe (i.e: cat | ./foo).

The problem is that programs like this will block all the fibers:

spawn do
  STDIN.gets
end

We used to have problems with stdio (#2713) but it was fixed by not changing the blocking mode from the original file descriptor (#6518)

This PR adds a third state to the blocking argument of FileDescriptor.new. When a nil is passed (the default) it detects the type of descriptor and sets the blocking mode accordingly.

Unfortunately this is quite difficult to test, so there is no specs to find regressions. I added a spec to check the STDIN opens in different modes depending on the fd type. But please, help me find if I didn't miss anything and I'm actually breaking everything again 😅

[Sija]

Couple of pedantic suggestions

[waj]

Couple of pedantic suggestions

Love all the suggestions. You know the language better than me :)