Do not replace /dev/null with a regular file in WriteFile

[zakinko]

WriteFile unlinks its target before creating it with O_CREAT. The unlink is
there so a running executable can be replaced, since you cannot open one with
O_TRUNC; it has been in the function since the first import, when WriteFile's
job was to write executables: the open used mode 0755 and the comment said so.

//src/test/cpp/util:file_test writes to /dev/null, in TestWriteFile and again
with a four-gigabyte NUL-filled buffer in TestLargeFileWrite. As an ordinary
user the unlink fails and the write reaches the device, so nothing happens.
As root it succeeds and the node becomes a regular file.

On GitHub's ubuntu-latest runner, where /dev is devtmpfs, at 870abbb,
running the test as root:

before   /dev/null  character special file  crw-rw-rw-  1:3
//src/test/cpp/util:file_test    PASSED
after    /dev/null  regular file            -rw-r--r--  0:0  size=215

Debian 13 in a container and NetBSD 11.0/amd64 do the same. On NetBSD,
polling once a second while TestLargeFileWrite runs:

t=7  Character Device  size=0
t=8  Regular File      size=4000000000
t=9  Regular File      size=0

/dev holds a four-gigabyte file for about a second, until the next thing that
redirects to /dev/null truncates it.

What follows the missing device is harder to find than the device itself:
2>/dev/null becomes a write, and the next program to read /dev/null gets
whatever was left there.

file_windows.cc already returns early for /dev/null. This is the same three
lines on the POSIX side. The POSIX test then writes to /dev/null and checks
that it is still a device: as an ordinary user that cannot fail, and as root
it fails there instead of quietly corrupting everything that runs after it.

without the change, as root   file_posix_test.cc:336  Expected: true   FAILED
with the change, as root      PASSED, /dev/null still 1:3

file_test passes as an ordinary user on ubuntu-latest, macos-latest and
windows-latest (the Windows build does not compile either file touched here).
The tests that fail as root do so before and after alike:
FilePosixTest.CanAccess and FileTest.TestRemoveRecursivelyPosix.

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Updated for 387d25a62bacb37fe62dd69b59d2af2aac450fb5