Support non-stdin pty/pts devices: Assign the correct IO to the stty calls

[byteit101]

The current stty size calls implicitly assume the device to read from is stdin, which is not overridden when @rl_instream is manually set to a pts device that isn't mapped to $stdin. As such, when $stdin is a pipe or other device where $stdin.tty? == false, the prompt and history editing is not visible. Further, if $stdin is sized differently than @rl_instream, the size will be erroneously for $stdin and not @rl_instream.

This PR:

• Adds a new private method io_exec that correctly calls IO.popen with @rl_instream as stdin
• Replaces all calls to backtick-launched processes with io_exec
• Uses @rl_instream in the search-exit logic instead of hardcoding $stdin
• Adds a new test to test that pty/pts devices correctly receive the prompt and visual editing. Fails on current master, passes with the other changes enclosed.

I have successfully tested this on JRuby 9.2, MRI 2.2.0, MRI 2.5.1, MRI 2.6.2 on RHEL 7.6 & Debian 9.

I have not tested this on Windows or macOS, and would appreciate anyone with such a machine testing this change, as I have limited access to such boxes.