[fdio] Reduce usage of 'misc' operation.

Rather than relying on an ioctl-like 'misc' function
which acted as a bottleneck for both remote and
socket operations, split operations into separate
optional function pointers.

This change provides type safety to remote operations,
and makes it less cumbersome to add / modify these
ops.

Additionlly, fix an fd leak in futimens.

Test: Expanded fdio tests, fs tests

Change-Id: I78f0c2a4291636b35727a9bcbfeaa4e67fdcbce1

[fdio] Support F_GETFL on connected socket FD.

This allows the consumer to set the O_NONBLOCK flag on a file
descriptor created from a connected socket (no control plane).

TEST: fdio-test
Change-Id: Id2370cfa9d3a4b080e6914684955c2a4e21ab77c

[fdio] Remove superfluous handle check.

When called by fdio_create_fd with a zx::socket handle, the control
channel handle will be invalid. Since this just closes the handle the
INVALID handle is fine without the check.

TEST: fdio-test
Change-Id: I0ffb2bc0868cfb27b0330c4c037a50c8d967c3b8

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

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[fdio] Unwrap secondary handles from PA_FDIO_REMOTE

When file descriptors are transferred via "fdio_transfer_fd",
they acquire a PA_FDIO type, which causes information loss about the
type of the control channel. Without information about the protocol
spoke over the control channel, we can still create a remote object,
but we will not know the underlying type.

Long-term, we will only transfer the control channel, but short-term,
this patch unbreaks callers of "fdio_create_fd".

ZX-1782 #comment Workaround

Change-Id: I219b5932b1b384623c8b722074bc2de54e76f786

[fdio] Add ppoll implementation

This allows for shorter timeouts than poll, and is used by
libvulkan_arm. poll's implementation now calls into ppoll.

Change-Id: I3c2cb46eaabcda0d1a22a607426fd0186ee535f2

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94