Clean: Actually use HvUSEDKEYS() instead of HvKEYS()

This:

  commit 8aacddc1ea3837f8f1a911d90c644451fc7cfc86
  Author: Nick Ing-Simmons <nik@tiuk.ti.com>
  Date:   Tue Dec 18 15:55:22 2001 +0000

      Tidied version of Jeffrey Friedl's <jfriedl@yahoo.com> restricted hashes
       - added delete of READONLY value inhibit & test for same
       - re-tabbed

      p4raw-id: //depot/perlio@13760

essentially deprecated HvKEYS() in favor of HvUSEDKEYS(); this is
explained in line 144 (now 313) of file `hv.h':

  /*
   * HvKEYS gets the number of keys that actually exist(), and is provided
   * for backwards compatibility with old XS code. The core uses HvUSEDKEYS
   * (keys, excluding placeholdes) and HvTOTALKEYS (including placeholders)
   */

This commit simply puts that into practice, and is equivalent to running
the following (at least with a35ef416833511da752c4b5b836b7a8915712aab
checked out):

  git grep -l HvKEYS | sed /hv.h/d | xargs sed -i s/HvKEYS/HvUSEDKEYS/

Notice that HvKEYS is currently just an alias for HvUSEDKEYS:

  $ git show a35ef416833511da752c4b5b836b7a8915712aab:hv.h | sed -n 318p
  #define HvKEYS(hv)		HvUSEDKEYS(hv)

According to `make tests':

  All tests successful.

1 files changed, 1 insertions, 1 deletions

diff --git a/mg.c b/mg.c
index d061c51d7f..cfa319728f 100644
--- a/mg.c
+++ b/mg.c
@@ -1732,7 +1732,7 @@ Perl_magic_getnkeys(pTHX_ SV *sv, MAGIC *mg)
     if (hv) {
          (void) hv_iterinit(hv);
          if (! SvTIED_mg((const SV *)hv, PERL_MAGIC_tied))
-	     i = HvKEYS(hv);
+	     i = HvUSEDKEYS(hv);
          else {
 	     while (hv_iternext(hv))
 	         i++;