Fix readline request locals

Summary: readline was storing request local values in
global variables, leading to bizarre crashes when used
across multiple requests.

Reviewed By: @edwinsmith

Differential Revision: D1669798

Signature: t1:1669798:1415638941:c520bacb8b349a88f33a4bb0af2ae1abceeb7a10
Use a static string for the function name in zend_register_internal_class

Summary: The table out-lives the request, and request-local strings are not
safe to share.

Reviewed By: @markw65

Differential Revision: D1669627