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

The libc crate is used as a dependency of the Rust compiler. Its build
system passes `--cfg dox` to all crates when generating their
documentation. libc's documentation is generated when the build system
is asked to generate the compiler documentation because `cargo doc`
automatically documents all dependencies.

When the dox configuration option is enabled, libc disables its
dependency on the core crate and provides the necessary definitions
itself. The dox configuration option is meant for generating
documentation for a multitude of targets even if the core crate for that
target is not installed. However, when documenting the compiler, it's
not necessary to do that; we can just use core or std as usual.

This change is motivated by the changes made to the compiler in
rust-lang/rust#48171. With these changes, it's necessary to provide
implementations of the Clone and Copy traits for some primitive types in
the library that defines these traits (previously, these implementations
were provided by the compiler). Normally, these traits (and thus the
implementations) are provided by core, so any crate that uses
`#![no_core]` must now provide its own copy of the implementations.

Because libc doesn't provide its own copy of the implementations yet,
and because the compiler's build system passes `--cfg dox` to libc,
generating the documentation for the compiler fails when generating
documentation for libc. By renaming the configuration option, libc will
use core or std and will thus have the necessary definitions for the
documentation to be generated successfully.

This follows the same method as other platforms like OSX and NetBSD.

This will fix rustup and building from git (once libc is updated for bootstrap)
on FreeBSD12 post-ino64 in https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a.
It also avoids having to hotpatch the stage0 compiler, and HOME/.cargo
libc files on FreeBSD12 to build rust.

The only real pitfall is that this will prevent interaction with inodes that
have an ino_t above the 32-bit limit due to truncation.  On the other hand
Rust won't work at all on 12 without doing this currently.  In general
it should not be a problem for users and if they need 64-bit ino_t they
can use a patched libc, rather than the current state of affairs in
requiring a patched libc to use Rust on 12.

A better, or complementary, approach would be something like proposed in
https://github.com/rust-lang/rfcs/pull/2048 to allow targetting a specific
version of FreeBSD. This would allow Rust to default to this compatibility
mode by targetting FreeBSD10 and still allow targetting FreeBSD12 for 64-bit
ino_t.

The symbol versions used were taken from the old version in
https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a#diff-61a32fcfb7ecd4517665fed591813c57
and
https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a#diff-7f67ccf8b5f44ff2f54eaab0207abb8d.

The scope of functions versioned here differs from other platforms as
not all structs were modified that were on others, such as DIR for
`opendir`, `telldir`, etc.  Only functions using dirent, stat, glob_t,
and dev_t need the changes.

Fixes https://github.com/rust-lang/rust/issues/42681

The `d_type` field of `struct dirent` can be `DT_UNKNOWN` if a
filesystem doesn't support returning the file type in directory entries.
Linux's readdir(3) man page, for example, says: "Currently, only some
filesystems [...] have full support for returning the file type in
d_type. All applications must properly handle a return of DT_UNKNOWN."

This reverts commit f816434f.

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

It's not supported on those platforms

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

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

Add missing fexecve() - execute program specified via file descriptor.

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html