This reverts commit 92a116c4ef822950f8c57eaa5164c844c73a1f7e with a fix
for fgets test - convert nullptr to fgets return type (char*), since the
matcher is pedantic.

Reverts llvm/llvm-project#143972 - matcher seems to be pedantic for
fgets tests, reverting to verify and fix.

This reverts commit a93e55e57ed00a55f822c64e3520c7c732b58480 and fixes
build and test failures:

* Proper include added to setvbuf_test.cpp
* fgetc/fgetc_unlocked/fgets tests are ported to ErrnoSetterMatcher and
are made more precise. This fixes inconsistencies between expectations
in regular and GPU builds - ErrnoSetterMatcher is configured to omit
errno matching on GPUs, as fgetc implementation on GPU doesn't set
errno, in contrast to Linux.

Reverts llvm/llvm-project#143802. Follow-up fix
3c7af175e51c3ab08ac3c442146c2b822f38c01e wasn't robust enough and itself
got reverted.

Reduce the direct use of libc_errno in stdio unit tests by adopting
ErrnoCheckingTest where appropriate.

Also removes the libc_errno.h inclusions from stdlib.h tests that were
accidentally added in d87eea35fac5a34a841c637db8908128409a184e

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

Much of unistd involves modifying files. The tests for these functions
need to use libc_make_test_file_path which didn't exist when they were
first implemented. This patch adds most of unistd to the bazel along
with the corresponding tests. Tests that modify directories had to be
disabled since bazel doesn't seem to handle them properly.

This patch provides specific test macros to deal with `errno`.
This will help abstract away the differences between unit test and integration/hermetic tests in #79319.
In one case we use `libc_errno` which is a struct, in the other case we deal directly with `errno`.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079