Installed-header hygiene (BZ#20366): stack_t.

Installed-header hygiene (BZ#20366): stack_t.

sys/ucontext.h unconditionally uses stack_t, and it does not make
sense to change that.  But signal.h only declares stack_t under
__USE_XOPEN_EXTENDED || __USE_XOPEN2K8.  The actual definition is
already in a bits header, bits/sigstack.h, but that header insists on
only being included by signal.h, so we have to change that as well as
all of the sys/ucontext.h variants.  (Some but not all variants of
bits/sigcontext.h, which sys/ucontext.h may also need, had already
received this adjustment; for consistency, I made them all the same,
even if that's not strictly necessary in some configurations.)
bits/sigcontext.h and bits/sigstack.h also all need to receive
multiple inclusion guards.

	* sysdeps/generic/sys/ucontext.h
	* sysdeps/arm/sys/ucontext.h
	* sysdeps/i386/sys/ucontext.h
	* sysdeps/m68k/sys/ucontext.h
	* sysdeps/mips/sys/ucontext.h
	* sysdeps/unix/sysv/linux/aarch64/sys/ucontext.h
	* sysdeps/unix/sysv/linux/alpha/sys/ucontext.h
	* sysdeps/unix/sysv/linux/arm/sys/ucontext.h
	* sysdeps/unix/sysv/linux/hppa/sys/ucontext.h
	* sysdeps/unix/sysv/linux/ia64/sys/ucontext.h
	* sysdeps/unix/sysv/linux/m68k/sys/ucontext.h
	* sysdeps/unix/sysv/linux/mips/sys/ucontext.h
	* sysdeps/unix/sysv/linux/nios2/sys/ucontext.h
	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h
	* sysdeps/unix/sysv/linux/s390/sys/ucontext.h
	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h
	* sysdeps/unix/sysv/linux/sparc/sys/ucontext.h
	* sysdeps/unix/sysv/linux/tile/sys/ucontext.h
	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h:
	Include both bits/sigcontext.h and bits/sigstack.h.
	Fix grammar error in comment, if present.

	* bits/sigstack.h
	* sysdeps/unix/sysv/linux/aarch64/bits/sigstack.h
	* sysdeps/unix/sysv/linux/alpha/bits/sigstack.h
	* sysdeps/unix/sysv/linux/bits/sigstack.h
	* sysdeps/unix/sysv/linux/ia64/bits/sigstack.h
	* sysdeps/unix/sysv/linux/mips/bits/sigstack.h
	* sysdeps/unix/sysv/linux/powerpc/bits/sigstack.h
	* sysdeps/unix/sysv/linux/sparc/bits/sigstack.h
	* bits/sigcontext.h
	* sysdeps/mach/hurd/i386/bits/sigcontext.h
	* sysdeps/unix/sysv/linux/bits/sigcontext.h
	* sysdeps/unix/sysv/linux/ia64/bits/sigcontext.h
	* sysdeps/unix/sysv/linux/sparc/bits/sigcontext.h:
	Add multiple inclusion guard.  Permit inclusion by sys/ucontext.h
	as well as signal.h, if this was not already allowed.  Request
	definition of size_t if necessary.  Minimize semantically-null
	differences across files.