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, 4 insertions, 4 deletions

diff --git a/dump.c b/dump.c
index 4e98394848..32e7596c63 100644
--- a/dump.c
+++ b/dump.c
@@ -1830,13 +1830,13 @@ Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 maxnest, bo
 	break;
     case SVt_PVHV:
 	Perl_dump_indent(aTHX_ level, file, "  ARRAY = 0x%"UVxf, PTR2UV(HvARRAY(sv)));
-	if (HvARRAY(sv) && HvKEYS(sv)) {
+	if (HvARRAY(sv) && HvUSEDKEYS(sv)) {
 	    /* Show distribution of HEs in the ARRAY */
 	    int freq[200];
 #define FREQ_MAX ((int)(sizeof freq / sizeof freq[0] - 1))
 	    int i;
 	    int max = 0;
-	    U32 pow2 = 2, keys = HvKEYS(sv);
+	    U32 pow2 = 2, keys = HvUSEDKEYS(sv);
 	    NV theoret, sum = 0;
 
 	    PerlIO_printf(file, "  (");
@@ -1878,13 +1878,13 @@ Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 maxnest, bo
             }
 	    while ((keys = keys >> 1))
 		pow2 = pow2 << 1;
-	    theoret = HvKEYS(sv);
+	    theoret = HvUSEDKEYS(sv);
 	    theoret += theoret * (theoret-1)/pow2;
 	    PerlIO_putc(file, '\n');
 	    Perl_dump_indent(aTHX_ level, file, "  hash quality = %.1"NVff"%%", theoret/sum*100);
 	}
 	PerlIO_putc(file, '\n');
-	Perl_dump_indent(aTHX_ level, file, "  KEYS = %"IVdf"\n", (IV)HvKEYS(sv));
+	Perl_dump_indent(aTHX_ level, file, "  KEYS = %"IVdf"\n", (IV)HvUSEDKEYS(sv));
 	Perl_dump_indent(aTHX_ level, file, "  FILL = %"IVdf"\n", (IV)HvFILL(sv));
 	Perl_dump_indent(aTHX_ level, file, "  MAX = %"IVdf"\n", (IV)HvMAX(sv));
 	Perl_dump_indent(aTHX_ level, file, "  RITER = %"IVdf"\n", (IV)HvRITER_get(sv));