[PATCH] Sv_*set() doc's and extra const's for the SvPVX_const() tasks

The attached patch finishes up the previous task for cleaning up lvalue 
assignments to Sv*X() macros and converting to the Sv*_set() macros.
Although most of the macros have been available since 5.003 at least, 
only one has been previously documented.  This patch takes care of that.  
Also, I wasn't exactly sure on the wording for the doc's, but if its off, 
I'll be glad to fix and resubmit.

The second part of this patch adds the SvPVX_const() macro in preparation for
conversions to that macro next week.

Finally, on place that showed up in early testing to require const'ing
from SvPVX_const() was the Perl_keyword() function in toke.c.  This patch
includes changes from embed.fnc, perl_keyword.pl, and toke.c to const up
that function.

Comments, questions, and concerns, as always, are welcomed.

Steve Peters
[email protected]