When stdin is piped (not a TTY), read the content and treat it as the
first message. Since interactive mode requires a TTY for keyboard input,
print mode is automatically enabled.
- echo foo | pi -> equivalent to pi -p foo
- echo foo | pi -p -> equivalent to pi -p foo
- RPC mode unaffected (uses stdin for JSON-RPC)
fixes#708
Mario Zechner
c50bfec01b