Add consts from netinet/ip.h

fix size_of function in dox.rs

I'm assuming this is meant to have the same signature as the one in core/std

Add linux socket ioctl defs

Moves `ifaddrs` and some functions to common Linux module

The `ifaddrs` interface is available since api version 24 in android.
The function signatures are now equal to the standard Linux function
signatures.

The `ifaddrs` interface is available since api version 24 in android.
The function signatures are now equal to the standard Linux function
signatures.

Add SIG* on redox

Add Redox definitions for net functions

Adds the following function definitions for Redox:

- getpeername
- sendto
- send
- recvfrom
- recv

If it is possible to do a release after merging this, that would be great! These function definitions are required for porting mio and tokio to Redox

android: add POSIX sysconf constants

Ref: https://android.googlesource.com/platform/bionic/+/1c19194c9d2518dbe86973cd313a277ecb70d75c%5E%21/libc/include/sys/sysconf.h

Ref: https://android.googlesource.com/platform/bionic/+/1c19194c9d2518dbe86973cd313a277ecb70d75c%5E%21/libc/include/sys/sysconf.h

Bump to 0.2.41

I mainly need the change that makes the fields of `nlmsghdr` public (c8e9232f).

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.