returncode should match return value.

Fix a few spots where the macaroon_returncode err does not match the
actual status of whether the operation was successful.

Change MACAROON_SUCCESS to 0, so that non-zero always indicates error.
Fixes per rescrv, pull #11

Pull returncode conditional up from recursive function macaroon_verify_inner to
non-recursive macaroon_verify_raw, to prevent timing attacks.

Revert MACAROON_SUCCESS enum value.
Merge branch 'rc-fixes-upstream' of git://github.com/cmars/libmacaroons

* 'rc-fixes-upstream' of git://github.com/cmars/libmacaroons:
  Fixes per rescrv, pull #11
  returncode should match return value.