Implement error handler function's context parameter

Summary: Closes https://github.com/facebook/hhvm/pull/3409

Reviewed By: @ptarjan

Differential Revision: D1492036

Pulled By: @sgolemon
Add Extension::moduleEnabled() hook

Summary: This takes the special cases for
apc, apache, and xhp out of extension.cpp
and allows EZC extensions to be compiled in,
but not actually enabled.

Reviewed By: @ptarjan

Differential Revision: D1528920