These constant can be used to determine the maximum number of iovecs can
be passed to functions like readv/writev.

Linux like uses UIO_MAXIOV, while the BSD family uses IOV_MAX.

Previously clock_getcpuclockid was enabled only on linux, with this
change it is enabled on all linux_like and freebsdlike.

Linux defines a waitid type `P_PIDFD`, for use with process file
descriptors (`pidfd`). Add that constant.

In libc-test, add linux/wait.h to the Linux-specific and
Android-specific headers, to get the definition. Exclude it on Android
and musl for now, though, as the versions in CI don't have it yet.

On Linux, `sys/wait.h` defines a `W_STOPCODE` macro to construct a stop
code from the signal number of a stopping signal. Provide an equivalent
function.

Suggested-by: Ivan Tham <pickfire@riseup.net>

The SIGCHLD variant of the siginfo structure also provides fields for
user and system time; expose those as well.

The first 3 fields of `siginfo_t` have different orders on MIPS. When
casting `siginfo_t` to a different type to access the fields of the
`sifields` union, avoid giving names to the first three fields, since
they're only present for memory layout and shouldn't be accessed from
the casted structure type.

On Linux, siginfo_t cannot expose these fields directly due to
https://github.com/rust-lang/libc/issues/716 , so expose them as
functions, just like si_addr and si_value.

In order to get alignment correct on both 32-bit and 64-bit
architectures, define an sifields union that includes a pointer field,
to ensure that it has the same alignment as a pointer.

These constants appear in the si_code field of a SIGCHLD signal or
waitid-returned siginfo value.

On Linux, `sys/wait.h` defines a `W_EXITCODE` macro to construct an exit
code from a return value and a signal number. Provide an equivalent
function.

Co-authored-by: Yuki Okushi <huyuumi.dev@gmail.com>

https://man7.org/linux/man-pages/man3/clock_getcpuclockid.3.html

In Linux we replaced VMADDR_CID_RESERVED with VMADDR_CID_LOCAL
in commit ef343b35d46667668a099655fca4a5b2e43a5dfe.
It is available since Linux v5.6, and it can be used to do
local communication if supported.

This patch deprecates VMADDR_CID_RESERVED for backward
compatibility.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

As with the other *64 functions in Linux, `posix_fadvise64` is like
`posix_fadvise` but uses `off64_t` instead of `off_t`.

This ended up just getting copied from Linux as it appears to be the
same.

In particular:
Add timerfd constants and functions, from sys/timerfd.h.
Add EFD_SEMAPHORE and group all EFD_ constants together.
Add sigtimedwait function, from signal.h.
Add missing fallocate constants and functions, from linux/falloc.h and fcntl.h.
Add xattr functions, from sys/xattr.h.
Add SCHED_ and SEEK_ constants, from linux/sched.h and bits/seek_constants.h.
Add rlimit functions, from sys/resource.h.
Add RENAME_ constants, from stdio.h.
Add ino64_t type, from sys/types.h.

This commit defines ETIMEDOUT, SIGSTKSZ, MINSIGSTKSZ

Reorganized some constants as well so they're grouped a bit better.