O_SEARCH test: mark revokex an expected fail on NFS

The revokex test does not work when the scratch directory is created on NFS.
Given the nature of NFS, it likely can never work without looking like a
security hole since O_SEARCH would rely on the server knowing that the
directory did have +x at the time of open and that it's OK for it to have
been revoked based on POSIX specification for O_SEARCH.

This does mean that O_SEARCH is only partially functional on NFS in general,
but I suspect the execute bit getting revoked in the process is likely not
common.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D23573

O_SEARCH test: drop O_SEARCH|O_RDWR local diff

In FreeBSD's O_SEARCH implementation, O_SEARCH in conjunction with O_RDWR or
O_WRONLY is explicitly rejected. In this case, O_RDWR was not necessary
anyways as the file will get created with or without it.

This was submitted upstream as misc/54940 and committed in rev 1.8 of the
file.

O_SEARCH tests: plug trivial fd leak

Coverity correctly reports this as a resource leak. It's an admittedly minor
one, but plug it anyways.

This has been submitted upstream as misc/54939.

CID: 978288

Provide O_SEARCH

O_SEARCH is defined by POSIX [0] to open a directory for searching, skipping
permissions checks on the directory itself after the initial open(). This is
close to the semantics we've historically applied for O_EXEC on a directory,
which is UB according to POSIX. Conveniently, O_SEARCH on a file is also
explicitly undefined behavior according to POSIX, so O_EXEC would be a fine
choice. The spec goes on to state that O_SEARCH and O_EXEC need not be
distinct values, but they're not defined to be the same value.

This was pointed out as an incompatibility with other systems that had made
its way into libarchive, which had assumed that O_EXEC was an alias for
O_SEARCH.

This defines compatibility O_SEARCH/FSEARCH (equivalent to O_EXEC and FEXEC
respectively) and expands our UB for O_EXEC on a directory. O_EXEC on a
directory is checked in vn_open_vnode already, so for completeness we add a
NOEXECCHECK when O_SEARCH has been specified on the top-level fd and do not
re-check that when descending in namei.

[0] https://pubs.opengroup.org/onlinepubs/9699919799/

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D23247

netbsd-tests: libc: use correct modes in O_SEARCH tests

The current code clearly intended for these to be octal based on the values
used, but the octal prefix was forgotten. Add it now for correctness, but
note that we don't currently execute these tests.

This has been submitted upstream as misc/54902, so I've omitted the standard
FreeBSD markers that we tend to put into netbsd-tests for upstream-candidate
identification.

Reviewed by: ngie
MFC after: 3 days