ipcz: Clean up Get() API

This changes how IPCZ_GET_PARTIAL behaves. Data is still copied out of
the parcel as the caller's supplied buffer capacity allows, but the
parcel's own data contents are no longer consumed by the operation: a
subsequent Get() on the same parcel with the same output buffer size
will see the same data. Copied handles are still consumed by the call.

Additionally a Get() on a portal will now always dequeue its parcel
even if it wasn't fully consumed by the call. This simplifies the
implementation and specified behavior slightly.

With the new IPCZ_GET_PARTIAL behavior, a caller can dequeue a parcel
and simultaneously acquire a handle to it without affecting its
contents. This renders IPCZ_GET_PARCEL_ONLY redundant and it is
therefore removed here.

Bug: 1451625
Change-Id: I039b3c46812204ab86a802a2019793da641cebec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4595577
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/main@{#1154622}