This reverts commit 920cfeac.

These are flags required to implement the linux kernel loading mechanism.
Specifically finit_module.

they doesn't exists on OpenBSD. put them under NetBSD specific tree.

It looks like this was a mistake of #930 but should be easy to fix!

Some structs for implementing af_packet(7)

No longer needed by the looks of it!

Previous commit

dcff1547
"libc: changes to ppc64le musl branch to support building of rust on Alpine"

has removed stuff from mod.rs and added it to some submodules, but
missed the aarch64 submodule.

This copies the stuff that that commit added to the x86_64.rs submodule
and puts it into aarch64.rs.

These are used in the socket2 library.

amend to add style changes
amend to add style changes

Fixes #1017

I moved it up to src/unix/mod.rs, as it's specified in POSIX.1-2008 and
appears to be implemented on every Unix-like system.

The symbol names on macOS appear similar to those for opendir; I found
them via the commands below. I tested the x86_64 version;
fdopendir$INODE64 worked as expected.

$ nm -arch x86_64 /usr/lib/system/libsystem_c.dylib | grep fdopendir
000000000007ea6d T _fdopendir
000000000002ba97 T _fdopendir$INODE64
$ nm -arch i386 /usr/lib/system/libsystem_c.dylib | grep fdopendir
00082d1e T _fdopendir
0002b528 T _fdopendir$INODE64$UNIX2003
00082d1e T _fdopendir$UNIX2003