no warning/error when xcache.so is loaded incorrectly

[moo]

it's sometimes easy to configure xcache.so with "extension=xcache.so" without having to know the extension_dir, but maybe due to the trick in XCache, error/warnings from other extension does not work, so it should be done in XCache side:

1. make sure version info shown together with other zend extensions
2. make sure xcache.so is loaded before other zend_extension if any. in other word, warn the user to use zend_extension=/path/to/xcache.so instead of extension=so when there is other zend extensions.