[fs] Ensure that cloned vnodes are not opened with O_PATH

ZX-2346 #done
Test: Added test_opath_fdcount to prevent regressions

Change-Id: I7a71dbd0f210b35a4d8f33c8f7938cc01c22d58c

[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

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] Help access tests not fail for filesystems without hard links

Change-Id: I4315fb3f8ae9b932d21a56e2c8262959b25b5dfa

[fs] Implement and test O_PATH

Although libfs will ensure invalid operations won't reach
the underlying filesystem, it is up to the implementation
of Vnode::Open to parse the O_PATH flag and construct
less state, if desired.

ZX-1151 #comment In Progress

Change-Id: I11f617b326f92b7ca03c3ec95d24db1e25ab2e59

[fs] Prevent opening directories as writable

Additionally, ensure (as POSIX dictates) that EISDIR
is returned for this case.

US-330 #done

Change-Id: I18abbb3a81b57755bdae7f627c61a11cae3b3b23

[utest] Remove empty-string msg args from ASSERT_*() calls in C++

The msg argument to ASSERT_*() is optional in C++ now, so we can drop
this argument when it's an empty string.

MG-905

Change-Id: I6a5d6917518f8c9c4865b761dcbefe296d54c924

[fs] Disallow invalid combinations of open flags

For example:
- O_WRONLY | O_RDWR
- O_RDONLY | O_TRUNC

Additionally, test this behavior.

Change-Id: Ib2d8ef1e0053475a76d51a0125abf1356a4a8bad

[fs] Add access checking for read, write, truncate

Change-Id: Ifde0659f4f230105be4444ed8710833174f548f8