Add dependency on headers with `in_addr` and `in_addr_t` type
definitions to ensure that these headers will be properly installed by
"install-libc" CMake target.

This patch provides definitions for `pkey_*` functions for linux x86_64.

`pkey_alloc`, `pkey_free`, and `pkey_mprotect` are simple syscall
wrappers. `pkey_set` and `pkey_get` modify architecture-specific
registers. The logic for these live in architecture specific
directories:

* `libc/src/sys/mman/linux/x86_64/pkey_common.h` has a real
implementation
* `libc/src/sys/mman/linux/generic/pkey_common.h` contains stubs that
just return `ENOSYS`.

This patch adds the posix function `inet_addr`. Since most of the
parsing logic is delegated to `inet_aton`, I have only included some
basic smoke tests for testing purposes.

These functions should be declared in `stdlib.h`, not `wchar.h`, as
confusing as it is. Move them to the proper header file and matching
directories in src/ and test/ trees.

This was discovered while testing libc++ build against llvm-libc, which
re-declares functions like mbtowc in std-namespace in `<cstdlib>`
header, and then uses those functions in its locale implementation.

These are simply implemented as specializations of strtofloatingpoint
for double / long double and for wchar_t. The unit tests are copied from
the strtod / strtold ones.

Reverts llvm/llvm-project#166355

_Exit(3) is a fairly simple syscall wrapper whereas exit(3) calls
atexit-registered functions + whole lot of stuff that require support
for sync primitives.

Splitting the tests allows testing the former easily (especially for new
port projects)

---------

Signed-off-by: Shreeyash Pandey <shreeyash335@gmail.com>

This change follows the pattern of
315dfe5865962d8a3d60e21d1fffce5214fe54ef by making strtofloat also
accept wchar_t* strings
(in addition to regular char*). It uses overloads from wctype_utils or
specialized functions to ensure comparison with literal characters (or
literal strings) pick char or wchar_t variants based on the argument
type.

The wcstof implementation is added, with unit test cases copied from
strtof test suite.

Closes #161461
- This is my first time contributing to libc's POSIX, so for reference I
used `clock_gettime` implementation for Linux. For convenience, here is
the description of `clock_settime` function
[behavior](https://www.man7.org/linux/man-pages/man3/clock_settime.3.html)

Implements fchown

fixes: #166856