varlink: support varlink communication via distinct input/output fds

varlink: support varlink communication via distinct input/output fds

When invoking another process via a pair of pipes it makes sense to
allow reading from one fd, and writing from another. Teach our varlink
code to do so optionally.

(sd-bus supports something similar, fill the gap).

This is preparation for a later commit that uses this to talk to remote
SSH invocations via pipes.