isatty() needs to work with other shells

[lundman]

Currently, isatty() is only working with a standard Windows CMDline box, to detect if zfs send is going to terminal, a file, or a socket.

This means you can not zfs send with: git for windows bash shell, mingw bash, cygwin bash etc.

isatty() needs to be enhanced to be able to work with all those combinations, implementations do exist in the wild, more a matter of finding the right combination.

Somewhat related to #17

[lundman]

b35a532