Make newlines in http headers a fatal error.

Summary: Make newlines in http headers a fatal error.

Reviewed By: @markw65

Differential Revision: D1720703
Define proper constants for CURLOPT and friends

Summary: I can never recall our inference story on constants, but we
don't infer these to ints, so explicitly declare as `const int`.

Reviewed By: @int3

Differential Revision: D1725225

Signature: t1:1725225:1418089362:d99d16add218d4b8dd8fdf37f7408c16c2065039