[fs][fidl] Replaced filesystem ioctls with FIDL.

This patch allows "ioctl" to be removed from the
C++ VFS Vnode interface.

IOCTL_VFS_MOUNT_FS -> Mount()
IOCTL_VFS_UNMOUNT_FS -> Unmount()
IOCTL_VFS_UNMOUNT_NODE -> UnmounteNode()
IOCTL_VFS_QUERY_FS -> QueryFilesystem()
IOCTL_VFS_MOUNT_MKDIR_FS -> MountAndCreate()
IOCTL_VFS_GET_DEVICE_PATH -> GetDevicePath()
IOCTL_VFS_WATCH_DIR -> WatchDir()

Test: fs-management / fs / blobfs / fvm tests, df manually

Change-Id: Iefe8ec251ca6b8f4e4578ceb61bae6dff7a59e3d

[fdio][fidl] Replace fuchsia-io hand-rolled client bindings

ZX-1919 #comment In Progress

Test: CQ (fs tests, fdio tests, blobfs tests, etc)

Change-Id: I31588bb62f5788fb9372fdc7042f42a3cae7c71c

[fdio][fidl][devmgr][fs] Deprecate Remote IO, enable FIDL2

Test: runtests (fs tests, fdio tests)
ZX-1919 #done

Change-Id: I459eb79546a3b952633faad2bb4914d9a3c68e05

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[fdio][fs] Remove usage of private headers in public libraries

ZX-1592 #done

Change-Id: I152d4164cb0a4d5e3cf74dfe231c234a78550f0f

[fs] Use the Fuchsia-defined open flags

Re-lands 2aff2cba029cf0f7e98e6d19f6dc7a663c318227

This change uses the Fuchsia-defined flags on the wire
as well as on filesystem servers. Clients interacting
with filesystems through the POSIX compatibility layer
can continue using POSIX open flags; they will be translated
by the RIO layers client-side.

ZX-1359 #comment In Progress

Change-Id: I4972c024547a0daa2146831e49e34d3ef905d4b4

[fs] Fix racy usage of remote handle

ZX-1161 #done

Change-Id: I520cc89f79c81524e025b93dfa4f6b7d3c2ffaa1

Revert "[fs] Use the Fuchsia-defined open flags"

This reverts commit 2aff2cba029cf0f7e98e6d19f6dc7a663c318227.

Reason for revert: Fuchsia uses upstream Rust, which doesn't pull these constants from our C library, but their own. I'll need to wait a few days for us to update upstream Rust before we can make progress here.

Original change's description:
> [fs] Use the Fuchsia-defined open flags
>
> This change uses the Fuchsia-defined flags on the wire
> as well as on filesystem servers. Clients interacting
> with filesystems through the POSIX compatibility layer
> can continue using POSIX open flags; they will be translated
> by the RIO layers client-side.
>
> ZX-1359 #comment In Progress
>
> Change-Id: I27dfdea2024183d5d59daa1ed7d8886d89d503d0

TBR=kulakowski@google.com,smklein@google.com,swetland@google.com,mcgrathr@google.com,planders@google.com

Change-Id: If93c2da36f92ea28ed013a61c01c6ec521da26cc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true

[fs] Use the Fuchsia-defined open flags

This change uses the Fuchsia-defined flags on the wire
as well as on filesystem servers. Clients interacting
with filesystems through the POSIX compatibility layer
can continue using POSIX open flags; they will be translated
by the RIO layers client-side.

ZX-1359 #comment In Progress

Change-Id: I27dfdea2024183d5d59daa1ed7d8886d89d503d0

[fs] Use fbl::StringPiece for libfs paths

ZX-1145 #done

Change-Id: I43e7b8c45e8d53d5c7cb90273b5eb57b2f9b4464

[fs] Separate Vnode, Vfs include files

Additionally, fix some headers, remove "ifdef cplusplus"
macros in ulib/fs (it isn't functional for C filesystems),
make the Readdir cookie typed, and make "MountNode" private
to the Vfs class.

MG-1117 #in-progress

Change-Id: Iba2169875b5dd73619f3e8c919fefb998a94a08f

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[fs] VFS-related cleanups

- RemoteContainer, WatchContainer moved to their own headers.
(Vnode be moved in a later patch).
- V_FLAG --> VFS_FLAG
- "vfs-XYZ.cpp" renamed to "XYZ.cpp" (headers too)
- WatcherBuffer (implementation detail) moved out of header.

MG-1117 #comment in-progress

Change-Id: I39471b142564d3d8bcf90a05949073d0fa1b32d3