sparc64-linux support

This needs to be "cleaned" up to use modules instead of a bunch of `cfg`s ...

Sadly, sparc64 constants are very different from other architectures so cleaning this will result in a bunch of duplication, I think.

While working on this, I was wondering why the constants are not written like this:

``` rust
// linux/mod.rs
const COMMON: ::c_int = 3;

cfg_if! {
    if #[cfg(target_arch = "sparc64")] {
        const FOO: ::c_int = 1;
    } else if #[cfg(any(target_arch = "mips64", target_arch = "x86_64"))] {
        const FOO: ::c_int = 2;
    } else {
        // unsupported/unknown architecture
    }
}
```

I think this might result in less duplicated code. @alexcrichton Has something like that ^ been attempted before?

Added initgroups

Not sure whether this is the correct platform level. I'll wait for CI to find out..

EDIT: Okay now it builds on all platforms but I think this configuration does not add the function on BSD where it has the same signature as on Linux.

Add execl, execle, execlp

I tried to be as close as possible to the existing definitions of `execv` and friends.

btw: do you still welcome these PR? Doesn't the [new bindgen/build.rs method](http://fitzgeraldnick.com/2016/12/14/using-libbindgen-in-build-rs.html) offer to automate this crate to 100%

Added constants from prctl.h

I left in the comment from prctl.h I can remove them if you want.

Add binding for mremap.

Please consider this PR.

Let me know if  you need things changed.

Add memalign to Redox (for use in allocator)

This is required to satisfy a review item here: https://github.com/rust-lang/rust/pull/38401

Added AF_UNSPEC and several consts from netdb.h

Fix doc generation on travis.

Documentation generation appears to be failing on master (https://travis-ci.org/rust-lang/libc/jobs/183483333):

```
Collecting ghp-import
  Downloading ghp-import-0.4.1.tar.gz
Collecting travis
  Could not find a version that satisfies the requirement travis (from versions: )
No matching distribution found for travis
```

Basically `--user` doesn't appear to take an option, so `pip install ghp_import --user $USER` makes pip think the user is a package that needs to be installed (in this case `travis`). As there is no `travis` package, it dies.

Documentation generation appears to be failing on master (https://travis-ci.org/rust-lang/libc/jobs/183483333):

```
Collecting ghp-import
  Downloading ghp-import-0.4.1.tar.gz
Collecting travis
  Could not find a version that satisfies the requirement travis (from versions: )
No matching distribution found for travis
```

Basically `--user` doesn't appear to take an option, so `pip install ghp_import --user $USER` makes pip think the user is a package that needs to be installed (in this case `travis`). As there is no `travis` package, it dies.

Add clock_settime()

See http://www.catb.org/esr/time-programming/#_clock_gettime_2_clock_settime_2_clock_getres_2

See http://www.catb.org/esr/time-programming/#_clock_gettime_2_clock_settime_2_clock_getres_2

When checking the status from waitpid on a kill -STOP <child_pid> WIFSIGNALED returns true

Currently in WIFSIGNALED rust is doing:
(status & 0x7f) + 1
where status is i32

As defined in /usr/include/x86_64-linux-gnu/bits/waitstatus.h
#define __WIFSIGNALED(status) \
  (((signed char) (((status) & 0x7f) + 1) >> 1) > 0)

Here is an example of the issue:
http://paste2.org/fXc8BxJ0

Run it, and it'll print the child pid then:
kill -STOP <child_pid>

Expect:
  Stopped by signal print statement
Results:
  Killed by signal print statement

Using the i32, it wont overflow leaving you with 128 returning true, using the waitstatus define you'll end up with -64 (since it shifts 1 right) which would return false. Though the C version shifts right once not really sure *why* but theres most likely a reason somewhere.

For the fix, just cast to i8 (signed char pretty much) as the C version is doing.

RUNNING ALL TESTS
PASSED 7356 tests

Add clearenv()

Add needed structs for fd passing

Dragonfly fixes 2016-12

This fixes libc again for DragonFly.

Recognize NetBSD/sparc64 as a generic 64-bit NetBSD