Don’t run the shell automatically when the psysh bin is included.

Fixes #48
Fix STDERR and STDIN while running as the CLI SAPI.

For some reason, the CLI SAPI doesn't define STDERR and STDIN constants, even though 'php://stderr' and 'php://stdin' still work like you'd expect.

This only affected people without Readline or Libedit. Seriously, folks, install PHP with some sort of Readline support. It is so. much. better.

Fixes #54
Work around complaints when initializing shell from CLI SAPI.