Remove fexecvpe from Haiku and MacOS/IOS where it's not implemented

I was going to remove the skip-check from `build.rs` to see if we might be able to enable the function check on at least some platforms, but I'll assume it doesn't work on any as has been suggested.

Closes #732.

It's not supported on those platforms

*   correct sigaction
*   correct stat, alias stat64
*   remove doubled statvfs definition

Openbsd kqueue

The PR supersede #613 to add exception on the testsuite.

they will be released with OpenBSD 6.2

OpenBSDs kqueue now has EV_RECEIPT and EV_DISPATCH.

I wrote a patch and the devs merged it into current:
https://marc.info/?l=openbsd-tech&m=149621427511219&w=2

This change extends the rust libc crate to provide the feature.

skip siginfo_t.si_addr type check on OpenBSD

the type changed from *c_char to *c_void in 6.1

the type changed from *c_char to *c_void in 6.1

Add getgrouplist(), add initgroups() to BSD platforms

Add MAP_ANONYMOUS to DragonFlyBSD and FreeBSD

pwd/grp: Add reentrant getgr functions and consolidate getpw functions

Commit messages sum it up.

Add some more missing pieces

 * some constants for mman
 * `WIFCONTINUED()` for NetBSD & OpenBSD

Add SEEK_DATA and SEEK_HOLE constants to FreeBSD and DragonFlyBSD

They can be found [here](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/725edadf86d63f56a584adf23265845c8590d734/sys/sys/unistd.h#L126) for DragonFlyBSD and [here](https://github.com/freebsd/freebsd/blob/f5d95e1f8d32db4ccccfd5ad9cecb21ed07a695d/sys/sys/unistd.h) for FreeBSD.

* Doesn't do much without getpwent/setpwent
* Not supported on Android before Android O

Add more socket flags for Android and the BSDs

L4Re builds don't have default libraries. The L4Re build system controls
compilation and passes linker flags using -Z itself.

Add some permission and misc (mostly 'mode_t' related) constants to Redox.

Hi!

While porting some software ((exa)[https://github.com/ogham/exa])to Redox I found these missing constants.

Cheers!

//cc @ids1024 @jackpot51

*   fix dirent, dirent64
*   port pthread_attr_t
*   add l4re-specific pthread code

struct addrinfo needs padding on Solaris/SPARC

64-bit Solaris/SPARC has a 4-byte pad before `ai_addrlen` for historical reasons (`ai_addrlen` used to be defined as a `size_t`, which is 4 bytes in ILP32 and 8 in LP64, but was converted to a 4-byte `socklen_t` in Solaris 10, which necessitated padding for (more or less) binary compatibility).

See rust-lang/rust#43649.