libc: make return value of isatty(3) user configurable

[Lukáš Zaoral]

Current implementation of isatty(3) in DiOS assumes that all valid file descriptors are terminal devices which is not generally true. It would be useful it the user could toggle this behaviour, e.g. all valid file descriptors would not be terminal devices.

At the moment Divine fails to verify gzip executed as part of its upstream test-suite only due to this hard-coded assumption as the tests do not expect any tty's. (However, the verification succeeded when we hard-coded 0 as the return value of isatty(3).)