Fix sys/ucontext.h namespace from signal.h etc. inclusion (bug 21457)

[Zack Weinberg]

On Mon, May 22, 2017 at 6:34 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> The various sys/ucontext.h headers include <signal.h> and all the
> headers split out of <bits/sigstack.h>.  (Except that the powerpc
> version does not include <signal.h>.)
>
> None of the standard versions defining ucontext.h require or permit
> such inclusions; rather, they all say that the stack_t and sigset_t
> types from signal.h are defined.  This patch fixes the headers to
> include just the bits/ headers for those types (and the existing
> includes of bits/sigcontext.h).

Looks good to me.