Apply simple optimisation for ${ subshare; } (re: 3d38270b)

Running shbench after undoing the incorrect subshell optimisation
showed that the performance of ${ subshare; }-type command
substitutions went down very slightly, but consistently.

The main purpose of using this ksh-specific type of command
substitution vs. a normal one is performance. Thus, it *is*
appropriate to eke every last bit of performance out of it that
we can, provided correctness is completely preserved.

It is also a type of command substitution where every change is
supposed to be shared with the main shell environment; only command
output is captured in a subshell-like fashion.

Thus, on the face of it, it would be a logical optimisation for
sh_assignok() to avoid bothering with saving a subshell context for
variables if we're in a subshare.

Lo and behold, applying it does not introduce any regress fails.

Here are my average results of the braces.ksh benchmark from
shbench <http://fossil.0branch.com/csb/tktnew> against stock
/bin/ksh 93u+ vs. current 93u+m (same compiler flags),
100 runs pre-optimisation and 100 runs post-optimisation:

Stock /bin/ksh:		Pre-optimisation (at 3d38270b):
93u+: 0.743 secs	93u+m: 0.739 secs

Stock /bin/ksh:		Post-optimisation (now):
93u+: 0.744 secs	93u+m: 0.726 secs

The left column shows only a small margin of error with 100 runs;
the right one shows a very small, but not insignificant difference.

However, these tests were not very rigorous with 100 runs each.
If anyone wants to do it properly, please report results to
korn-shell@googlegroups.com. I'm happy enough with this, though.

Thanks to Joerg van den Hoff for providing shbench, without
which it would not have occurred to me to try this.

src/cmd/ksh93/sh/subshell.c: sh_assignok():
- Don't bother if we're in a ${ subshare; }.

src/cmd/ksh93/sh/subshell.c

@@ -248,6 +248,9 @@ Namval_t *sh_assignok(register Namval_t *np,int add)
 	Namval_t		*mpnext;
 	Namarr_t		*ap;
 	int			save;
+	/* don't bother to save if in a ${ subshare; } */
+	if(sp->subshare)
+		return(np);
 	/* don't bother with this */
 	if(!sp->shpwd || np==SH_LEVELNOD || np==L_ARGNOD || np==SH_SUBSCRNOD || np==SH_NAMENOD)
 		return(np);