Streamline some shell state flaggery

src/cmd/ksh93/sh/args.c: sh_argprocsub():
- Save and restore state more efficiently by just saving and
  restoring all the state bits in one go using the
  sh_{get,set}state() macros, which are defined in defs.h as:
    #define	sh_getstate()	(sh.st.states)
    #define	sh_setstate(x)	(sh.st.states = (x))
  (and there is yet more evidence that it doesn't matter whether
  we use a 'shp->' pointer or 'sh.' direct access).

src/cmd/ksh93/sh/main.c: exfile():
- Remove a no-op 'sh_offstate(SH_INTERACTIVE);'. It was in the
  'else' clause of 'if(sh_isstate(SH_INTERACTIVE))' so if we get
  there, it is known that this flag is already off.
- To properly disable job control, we also have to save and restore
  the job.jobcontrol variable.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Remove some no-op flaggery from this highly performance-sensitive
  point in the code. Given the immediately preceding:
	volatile int	was_errexit = sh_isstate(SH_ERREXIT);
	volatile int	was_monitor = sh_isstate(SH_MONITOR);
  the following:
	sh_offstate(SH_ERREXIT);
	if(was_errexit&flags)
		sh_onstate(SH_ERREXIT);
  can be reformulated as:
	if(!(flags & sh_state(SH_ERREXIT)))
		sh_offstate(SH_ERREXIT);
  (IOW, if it was already on, don't turn it off and then on again)
  ...and the following:
	if(was_monitor&flags)
		sh_onstate(SH_MONITOR);
  can be removed; it's a no-op because it wasn't preceded by an
  sh_offstate() and if 'was_monitor' is true, this option is known
  to be on. (I considered they may have forgotten an 'sh_offstate'
  there like in the SH_ERREXIT case, but adding that causes several
  regressions in a shtests run.)

src/cmd/ksh93/include/defs.h:
- Remove comment that is evidently long outdated; there is not (or
  no longer) a Shscoped_t type defined anywhere, nor are these
  struct fields replicated in any other type definition.
- Add comment to clarify what the 'states' int in 'struct
  sh_scoped' is for.

src/cmd/ksh93/sh/main.c

@@ -402,7 +402,6 @@ static void	exfile(register Shell_t *shp, register Sfio_t *iop,register int fno)
 			buff.mode = SH_JMPEXIT;
 			sh_onoption(SH_TRACKALL);
 		}
-		sh_offstate(SH_INTERACTIVE);
 		if(sh_isoption(SH_MONITOR))
 			sh_onstate(SH_MONITOR);
 		sh_offstate(SH_HISTORY);