Remove __need_schedparam and __cpu_set_t_defined.

Remove __need_schedparam and __cpu_set_t_defined.

bits/sched.h has logic to expose only an impl-namespace variant of
struct sched_param (i.e. struct __sched_param), but nothing uses it,
and the only header that includes bits/sched.h is sched.h.  The
__need_schedparam logic can therefore be removed.

bits/sched.h also has a great deal of code relating to cpu_set_t
objects that was *almost* the same between the two versions of
bits/sched.h in the tree; a little spelunking indicated that this is
because some bug fixes got applied to the Linux-specific bits/sched.h
but not the generic one.  Introduce a new header, bits/cpu-set.h,
containing the version of that code with the bugfixes, have sched.h
include it directly, and delete all of the code from both versions of
bits/sched.h.

Also remove the unnecessary name mangling in the definition of struct
sched_param -- POSIX specifies a field 'sched_priority', so there is
no reason to define it as '__sched_priority' and then paper over that
with a macro.  (Just in case someone was using the internal name,
'sched_priority' remains a macro defined to expand to itself, and
'__sched_priority' now expands to 'sched_priority'.)

Finally, as long as I'm touching these files anyway, merge new
constants from linux/sched.h into the Linux bits/sched.h.

	* bits/sched.h: Remove __need_schedparam logic and replace with a
	normal multiple-include guard.  Change field name in struct
	sched_param from __sched_priority to sched_priority.  Delete
	everything under #ifndef __cpu_set_t_defined.
	* sysdeps/unix/sysv/linux/bits/sched.h: Likewise.  Also sync with
	kernel sched.h, adding SCHED_ISO and SCHED_DEADLINE constants.

	* posix/sched.h: Include bits/cpu-set.h as well as bits/sched.h.
	For compatibility, #define sched_priority to itself, and #define
	__sched_priority as sched_priority.
	* posix/bits/cpu-set.h: New file containing, verbatim, the code
	that was under #ifndef __cpu_set_t_defined in
	sysdeps/unix/sysv/linux/bits/sched.h.
	* include/bits/cpu-set.h: New wrapper.
	* posix/Makefile: Install bits/cpu-set.h.