std: move os/darwin.zig and related to c/darwin.zig

Move to c/darwin.zig as they really are libSystem/libc imports/wrappers.

As an added bonus, get rid of the nasty `usingnamespace`s which are now
unneeded.

Finally, add `os.ptrace` but currently only implemented on darwin.

Author: kubkon

CMakeLists.txt

@@ -296,7 +296,6 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/lib/std/meta/trait.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/multi_array_list.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os.zig"
-    "${CMAKE_SOURCE_DIR}/lib/std/os/darwin.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux/errno/generic.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux/x86_64.zig"