The previous internal fcntl implementation modified errno directly, this
patch fixes that. This patch also moves open and close into OSUtil since
they are used in multiple places. There are more places that need
similar cleanup but only got comments in this patch to keep it
relatively reviewable.

Related to: https://github.com/llvm/llvm-project/issues/143937

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

Fixes https://github.com/llvm/llvm-project/issues/126603

---------

Signed-off-by: ZakyHermawan <zaky.hermawan9615@gmail.com>

https://man7.org/linux/man-pages/man2/setsid.2.html

closes #124632

Implement GNU extension getentropy. This function is used by many
programs to acquire entropy without handling the loop of getrandom.

Reverts the revert #119295 of #118882 by expanding #118882 with
additional fixes which made CI unhappy.

Reverts llvm/llvm-project#118882

Several functions are now missing necessary types in fullbuild, e.g.
`off_t`, `ssize_t`. Reverting for now.

Fixes failures like this which I experienced:
```
FAILED: src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o
/nix/store/2r2xi5pbg29bsmqywsm5zgl8l7adky4i-gcc-wrapper-13.3.0/bin/g++ -DLIBC_NAMESPACE=__llvm_libc -D_DEBUG -I/build/libc-src-20.0.0-unstable-2024-12-05/libc -isystem /build/libc-src-20.0.0-unstable-2024-12-05/libc/build/include -g -std=gnu++17 -DLIBC_QSORT_IMPL=LIBC_QSORT_QUICK_SORT -DLIBC_ADD_NULL_CHECKS -DLIBC_COPT_PUBLIC_PACKAGING -MD -MT src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o -MF src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o.d -o src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o -c /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp
次のファイルから読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/OSUtil/linux/syscall.h:13,
         次から読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/OSUtil/syscall.h:15,
         次から読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:11:
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:20:29: エラー: ‘ssize_t __llvm_libc::read(int, void*, size_t)’ が外部シンボル ‘read’ の別名となっています
   20 | LLVM_LIBC_FUNCTION(ssize_t, read, (int fd, void *buf, size_t count)) {
      |                             ^~~~
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/common.h:46:34: 備考: in definition of macro ‘LLVM_LIBC_FUNCTION_IMPL’
   46 |   decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];                   \
      |                                  ^~~~
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:20:1: 備考: in expansion of macro ‘LLVM_LIBC_FUNCTION’
   20 | LLVM_LIBC_FUNCTION(ssize_t, read, (int fd, void *buf, size_t count)) {
      | ^~~~~~~~~~~~~~~~~~
```

Closes #85289

Co-authored-by: Michael Jones <michaelrj@google.com>