Allow specifying an input stream for commands expecting a TTY input

[ddimtirov]

Ideally we would want a DSL along the lines of 'expect', but as a very minimum, the ability to pump stuff through an input stream will allow us to run kinit.

[aadamovich]

Yes, that would be nice. Not quite sure yet how to implement it though

[nthykier]

Hi,

Admittedly the tty part is (currently a bit beyond me). I have made a sample patch in nthykier@32d2b62a for adding support for adding an "input" option to exec. It contains an UTF-8 encoded string that is to be the new process's stdin.

Sadly I was unable to make a test case for it. I wanted to do a "cat -" process in BaseSshTest that simply set output to be input, but all my attempts let to the test hanging.

If time permits, I will try to field test it on a "real" system, but for now it is just an untested patch.