Add new sendfile flags for FreeBSD

Two new constants for FreeBSD:
* `SF_USER_READAHEAD` introduced in FreeBSD 12 [in this commit](https://github.com/freebsd/freebsd/commit/b739d603442d3aa17f07d55918ca3a07eefa24f7#diff-b3f45d20f63d013e46f59f3f88a84d3d) (omitted from tests until FreeBSD 12 is stable)
* `SF_NOCACHE` introduced in FreeBSD 11 [in this commit](https://github.com/freebsd/freebsd/commit/aaa09777e1d9fde5591814af536025be01a0182f#diff-b3f45d20f63d013e46f59f3f88a84d3d)

Add sem_getvalue to unix

Adds `sem_getvalue` for supported platforms.

Reference: https://www.gnu.org/software/libc/manual/html_node/Semaphores.html

Add TIOCGWINSZ to openbsd

Adds a commonly used ioctl to openbsd, related to #957. I've tested this on OpenBSD 6.3.

add remaining pktinfo consts/structs for the other BSDs

This continues the work from https://github.com/rust-lang/libc/pull/980, adding a more complete set of supported platforms for the "I want to know what destination this UDP packet had" problem.

define more TCP_ sockopts on bsd

seee #978

sources for sockopt numbers:
- openbsd: i can't find them, so maybe they dont exist.
- dragonfly: https://github.com/DragonFlyBSD/DragonFlyBSD/search?utf8=%E2%9C%93&q=TCP_KEEPINTVL&type=
- netbsd: https://github.com/IIJ-NetBSD/netbsd-src/search?utf8=%E2%9C%93&q=TCP_KEEPINTVL&type=

Signed-off-by: Arvid E. Picciani <aep@exys.org>

add pktinfo consts and structs for linux and apple

The constants and structs are from `/usr/include/linux/in.h`, `/usr/include/linux/in6.h` for Linux (+ Android) and `/usr/include/netinet/in.h`, `/usr/include/netinet6/in6.h` for Apple.

With Apple it's important to mention that I'm choosing the RFC 3542 versions of the constants in alignment with other critical pieces of software like [nginx](http://hg.nginx.org/nginx/rev/9fb994513776), and Linux also defaults to this RFC version.

fix README.md links

Follow up to PR #977, which just fixed lib.rs while leaving similar broken links in README.md.

fix domain of html_root_url to be rust-lang.github.io

Fixes #976

haiku: Add RLIMIT_STACK and fix RTLD_* definitions

This commit adds additional definitions for Haiku which fixes issues in building libc and backtrace-rs when cross-compiling Haiku using docker ci.

Fix undefined-behavior on MacOSX structs in stdbuilds

Some MacOSX structs have an incorrect layout that results in undefined behavior. This is because on `x86_64` the MacOSX kernel headers define these using `#pragma pack 4`.

This PR fixes their layout using `repr(packed(4))` . Since it is only available on nightly, it is only enabled for stdbuilds .

feat: add fread_unulock for linux

add fread_unulock for linux

Make netlink struct fields public

I realized my netlink structs were a bit unusable if their fields are not public 

Follow up and improvement to #922

Refresh Cargo.lock

Fixes #968.

Fixes #968.

Add PTHREAD_*_MUTEX_INITIALIZER_NP for glibc

`pthread_mutex_t` varies across architectures, in several ways:
- endianness alters the ordering of bytes, since the contents of the
  struct are larger than 8-bit.
- its length varies.
- the location of the mutex kind (`PTHREAD_MUTEX_RECURSIVE`,
  `PTHREAD_MUTEX_ERRORCHECK` or `PTHREAD_MUTEX_ADAPTIVE_NP`) varies
  between 32-bit and 64-bit: On 32-bit architectures, it is preceded by
  three int/unsigned int, while on 64-bit architectures, it is preceded
  by four of them.

These initializers are only available from <pthread.h> when _GNU_SOURCE
is defined.

`pthread_mutex_t` varies across architectures, in several ways:
- endianness alters the ordering of bytes, since the contents of the
  struct are larger than 8-bit.
- its length varies.
- the location of the mutex kind (`PTHREAD_MUTEX_RECURSIVE`,
  `PTHREAD_MUTEX_ERRORCHECK` or `PTHREAD_MUTEX_ADAPTIVE_NP`) varies
  between 32-bit and 64-bit: On 32-bit architectures, it is preceded by
  three int/unsigned int, while on 64-bit architectures, it is preceded
  by four of them.

These initializers are only available from <pthread.h> when _GNU_SOURCE
is defined.

Relax the cfg_if check in ci/style.rs to allow #[cfg(target_endian)]
tests.

openbsd: SIGSTKSZ has been reduced

On OpenBSD 6.3, `SIGSTKSZ` value has changed.
https://github.com/openbsd/src/commit/e211c58cc8e20862a4eaa1b91fd4bc4a2975dad3

Add flock type value definitions for Linux/other/x86_64

Add MAP_STACK constant to OpenBSD

The mmap(2) flag indicate that the mapping is used as a stack.

https://man.openbsd.org/mmap.2#MAP_STACK
https://marc.info/?l=openbsd-tech&m=152035796722258&w=2