adb: create unix_isatty() function.

Our Windows code has several different ways it checks whether an FD is
a console or not, some of which aren't exactly correct as they may
treat character devices (e.g. NUL) as consoles.

This CL disallows using the builtin isatty() function and provides
unix_isatty() instead which correctly checks these corner cases.

Change-Id: I6d551c745dae691c7eb3446b585265d62c1e62fa