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.

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

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

sock_extended_err is a struct returned as a control message when the sockopt
IP_RECVERR is set, when recvmsg has the MSG_ERRQUEUE flag set.  IP_RECVERR and
MSG_ERRQUEUE are constants both already defined here.

This reverts commit 6ff2e8f1.

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Signed-off-by: patrick felt <patrick.felt@sling.com>

Add a new feature to enable this, since `const extern fn`
support is unstable

sigevent structs on most platforms have padding or unused fields. Rather
than display those in the Debug impl by deriving it, manually implement
all extra_traits instead ignoring those fields.

`nl_pad` field does not contain any actual data, so using it for
comparison or hashing doesn't make sense. Instead manually implement
extra traits ignoring this field.

Non-existent constants like SYS_syscalls have been removed

The second argument of `gettimeofday` was a `*mut c_void` on all targets,
but that type is incorrect in the following targets, where it should be
a `*mut timezone` instead:

On these other targets it appears that the signature of gettimeofday was incorrect (it takes a time-zone pointer instead of a void pointer):

linux+gnu: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
freebsd: https://www.freebsd.org/cgi/man.cgi?query=gettimeofday&apropos=0&sektion=2&manpath=FreeBSD+11.2-stable&arch=default&format=html
openbsd: https://man.openbsd.org/gettimeofday.2
android: https://github.com/ricardoquesada/android-ndk/blob/master/usr/include/sys/time.h
dragonfly: https://www.dragonflybsd.org/cgi/web-man?command=gettimeofday&section=2

This commit corrects the type on these targets, which is a breaking change. Due
to how this API is commonly used (e.g. passing `ptr::null_mut` to the second
argument), breakage should be minimal. Users wanting to support both versions
can just write `ptr as *mut _` instead.

Closes #1338.