sys.stdin.read() blocks waiting for EOF in shell.py which will
likely never come if the user provides input on stdin via the
keyboard before the shell starts. Added check for a tty to
skip reading stdin if it's not present.

This still allows piping of code into the shell (which should
have no TTY and should have an EOF) but also doesn't cause it
to hang if multi-line input is provided.

Backport of 4e78e389 from master.