Fix #275: don't use readline() if stdin is not a tty

Tarantool support three modes:

1. A script filename is passed to binary or server started using #!.
Tarantool read file contents and execute code as a whole chunk.
2. stdin is not a tty. Tarantool read stdin until EOF and execute
code as a whole chunk.
3. stdin is a tty. Tarantool starts in interactive mode and
processes console input line by line using dostring().

Please see the difference between #2 and #3.