varlink: add varlink_peek_dup_fd() helper

varlink: add varlink_peek_dup_fd() helper

This new call is like varlink_peek_fd() (i.e. gets an fd out of the
connection but leaving it also in there), and combines ith with
F_DUPFD_CLOEXEC to make a copy of it.

We previously already had varlink_dup_fd() which was a duplicating
version for pushing an fd *into* the connection. To reduce confusion,
let's rename that one varlink_push_dup_fd() to make the symmetry to
valrink_push_fd() clear so that we have no:

varlink_peer_push_fd()        → put fd in without dup'ing
varlink_peer_push_dup_fd()    → same with F_DUPFD_CLOEXEC
varlink_peer_peek_fd()        → get fd out without dup'ing
varlink_peer_peek_dup_fd()    → same with F_DUPFD_CLOEXEC