We should discourage/drop support for eaccelerator

Discovered from MDL-36060, eaccelerator doesn't support closures - a critical php 5.3 feature. Its also the way the php api is moving (if you could ever suggest a direction for php..)

https://github.com/eaccelerator/eaccelerator/issues/12

That feature is nearly 4 years old and that bug has been dead like that for nearly 10 months, which highly suggests eaccerator development is dead.

Unfortunately, eaccerator used to be much better than APC in the opcode cache world and so I imagine there are a lot of moodle sites using it.

I think that we should start with a warning and then also potentially drop support of eaccelerator (if we can detect it, not certain of that). If they aren't supporting anything as critical as closures, who knows what else could break.