libbinder: Stricter protocol and code for receiving FDs

This is a slight change the to wire protocol. Now out-of-band FDs must
be sent along with the command header bytes.

The code changes exploit that by only using the more complex `recvmsg`
call when reading the command header. Additionally, we explicitly pass
around the list of FDs so that there is no risk of accumulating them.

The same (somewhat ugly) vector type is used everywhere now so that
there is only one allocation to capture the FDs and pass them to the
`Parcel` object.

Test: binderRpcTest
Bug: 185909244
Change-Id: I1f55995ca82338ab9716fb2246c954ac8b16cfe5