Add getpid on Redox

None

add fdopendir on macOS

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

added renamex_np, renameatx_np

these function can be used to atomically swap files

- added 2 macos specific functions - `renamex_np` and `renameatx_np` and two constants `RENAME_SWAP` and `RENAME_EXCL`
  - [macos stdio.h](https://github.com/apple/darwin-xnu/blob/0ddccd8/bsd/sys/stdio.h#L45-L48)
  - [docs](http://www.manpagez.com/man/2/renameatx_np/osx-10.12.3.php)
- ~added linux specific `renameat2`. according to the [docs](http://man7.org/linux/man-pages/man2/rename.2.html#VERSIONS) it was added to Linux in kernel 3.15.~

add net/if_arp.h definitions

This is based on the contents of include/net/if_arp.h on my Linux system.

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

Add some termios constants for sparc64

Bump version to 0.2.42

Closes https://github.com/rust-lang/libc/issues/1011

make dox::mem::size_of a const fn

add more built-in traits to dox.rs

Add/fix bitwise operation traits in dox.rs

Fix sparc64 builds

Partially fixes #987.

add rtentry

Add `gethostname` support to Redox module

We need this for hostname support in Ion shell.

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