Adopt PHP7's looser handling of return values in user session handler

Summary: Explicitly handle 0/-1 return values and throw a warning
on anything but those or true/false.

Per #3055

Reviewed By: @paulbiss

Differential Revision: D1673400

Signature: t1:1673400:1415737574:61b8900d99dcffb36f1aec0ac3a4c3b3c75a3e1f