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

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[fs] Ensure Vnode::Close is called on unclean teardown

The contract of ulib/fs is that every call to "Open"
will be paired with an accompanying call to "Close",
in the event of either:

1) A client sends a close message explicitly, making
their channel defunct (and no longer observed), or
2) A client closes their channel, implicitly closing
their connection to the filesystem.

Although the Connection class in ulib/fs would properly
cancel waits and tear down the connection object,
it previoulsy missed sending a close in the second
condition, since it invoked "CallHandler" while the
channel was not yet ZX_HANDLE_INVALID.

This patch ensures that the close handler is invoked
in this condition.

ZX-1853 #done

Change-Id: I291192cae32e8250f7cef932cbf1a4de4b8b2d2d

[fs][test] Pass real buffer to read, even when we expect failure

Change-Id: I61ab253fd14a141f83d3745dfefa9ec92bc9ff6c

[fs] Fix fs tests now that read(fd, NULL, 0) is not an error

Change-Id: I2d16e633d6086447f2778472969358451f647f0f

[mxio][fs][remoteio] allow pipelined open()

Pull the pipeline flag into fcntl.h as O_PIPELINE to make it visible
to posix API consumers. Adjust mxio and fs implementations to use
this instead of the old remoteio constant. Use the same numeric
value to preserve wire format compatibility.

Allow O_PIPELINE to be passed to open() and cause a pipelined open
which will succeed as long as the message can be written, but will
result in failure (for unknown reason) when you attempt IO against
that object if the open was rejected.

Change-Id: I2a5be2a3cb99da16f9ab7662de9e98801dbf71cc

[magenta] Prevent opening directories as writable

Errno "EISDIR" for "open" can be returned for the following reason,
according to POSIX:
The named file is a directory and oflag includes O_WRONLY or O_RDWR.

(IEEE Std 1003.1, 2004 Edition)

Convert usage of opening a directory with "O_RDWR" or "O_WRONLY" to
"O_DIRECTORY" explicitly, when possible.

Change-Id: I47094798677dd22abcf2c40e3a1ff7b17525ba8c

[fs][minfs][memfs] Add support for unlinking open directories

- Update tests to allow these "empty, unlinked" directories.
- Update test suite to use "rmdir" instead of "unlink".
Exclusively for clarity; they functionally identical at the
moment.

MG-629 #done

Change-Id: I37024b873471639a83562fe9dfb68e18e4e1d3ee

[fs] Converted filesystem test suite from uapp --> utest

- This is possible now that we can identify "large" tests,
and prevent them from executing by default.

While I was there...
- I also fixed up the "test random op" condvar race contition
that previously plagued the concurrent "rw workers" test.
- Stdout spam reduced for the maxfile test.

Change-Id: I87fd25b6312d3490fa7842b9de22826646743d2a