add support for arbitrary file descriptors on unix

[m-mueller678]

This allows the use of arbitrary file descriptors on unix in response to issue #542. File Descriptors are supplied via a new variant of config::Bhaviour

[gwenn]

Could you please explain why only unix is supported ?
And I am afraid that this PR doesn't work for telnet / ssh connection because isatty will return false (jline3 seems to use the system flag to skip isatty check). So I guess we need something more complicated than a file descriptor...

[m-mueller678]

My main use case for this feature is working with pseudoterminals other than the one controlling the process.

Only Unix is supported because I don't have a Windows machine to work on and don't have any experience with Windows terminal handling. Not for any technical reason.

This does indeed not work well with things other than TTYs. I don't think you can reliably support anything that is not a TTY because you cannot change to raw mode without termios as far as I know. Do you think a flag to override isatty and just assume the user set everything up as needed would be a good idea? If so, Behavior::ArbitraryFileDescriptors seems like a good place to put it. Anyone using this should have to consciously make this choice, and the other behaviours likely don't need it.

As a side note: My current workaround for supporting ssh is to just hijack the pseudo terminal slave. If necessary, I send SIGSTOP to its controlling process to ensure it doesn't steal any of my input.