core: add deserialize_fd() helper + other minor fixes - #29481

Currently, when we deserialize an fd we do a lot of manual work. Add a
common helper that makes this more robust and uniform.

Note that this sometimes changes behaviour slightly, but in ways that
shouldn't really matter: if we fail to deserialize an fd correctly we'll
unset (i.e. set to -EBADF) the fd in the deserialized data structure.
Previously, we'd leave the old value in place.

This should not change effective result (as in either case we'll be in a
bad state afterwards, just once we mix old/invalidated state with new
state, while now we'll reset the state explicitly to invalidated state
on failure). In particular as deserialization starts from an empty
structure generally, hence the old value should be unset anyway.

Another slight change is that if we fail to deserialize some object half
way, and we already have taken out one fd from the serialized fdset
we'll now just close it instead of returning it to/leaving it in the
fdset. Given that such "orphaned" fds are blanket closed after
deserialization finishes this also shouldn't change behaviour IRL.

Also, the idle_pipe was previously incorrectly serialized: we'd
serialize invalidated fds, which would fail, but because parsing errors
on this were ignored on the deserializatin noone noticed. This is fixed.