Add cfg_attr

Closes #659

PR #849 just moved all of the Linux structures into `src/fuchsia`.
While this is good for the most part, we really don't want the `SYS_*`
constants exported on Fushia, as these are not Fuchsia's syscalls.

This also removes Linux-specific `GRND_*` constants.

sigevent structs on most platforms have padding or unused fields. Rather
than display those in the Debug impl by deriving it, manually implement
all extra_traits instead ignoring those fields.

* Change the type of FIONCLEX on apple platforms from c_uint to c_ulong
* Add FIONCLEX, FIONREAD, FIOASYNC, FIOSETOWN and FIOGETOWN
  for DragonFly and OpenBSD

During the refactor in 4bd419eb, PTRACE_DETACH was accidentally removed on
these targets. This re-adds it.

Add struct sockaddr_vm and VMADDR_ constants to use
VSOCK on Linux. VSOCK is present since Linux 3.9

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>