Fix stub for Phar::setStub()

Fix stub for Phar::setStub()

This fixes multiple issues:
 * The first parameter may be resource|string.
 * It's an overloaded signature. The second parameter cannot be
   passed if the first one is a string. Use UNKNOWN default
   value for that reason.
 * Make parameter names in PharData::setStub() match those in
   Phar.

Closes GH-6596.