locale.c: Simplify S_category_name

We can use the new function S_get_category_index() to simplify this.
Also, when I wrote it I didn't know about Perl_form(), and had
reimplemented a portion of it here; which is yanked as well.

locale.c

@@ -311,6 +311,7 @@ S_get_category_index(const int category, const char * locale)
      * elements is so far at most 12 */
 
     unsigned int i;
+    const char * conditional_warn_text = "; can't set it to ";
 
     PERL_ARGS_ASSERT_GET_CATEGORY_INDEX;
 
@@ -330,12 +331,16 @@ S_get_category_index(const int category, const char * locale)
     }
 
     /* Here, we don't know about this category, so can't handle it. */
+
     if (! locale) {
-        locale = "(unknown)";
+        locale = "";
+        conditional_warn_text = "";
     }
+
+    /* diag_listed_as: Unknown locale category %d; can't set it to %s */
     Perl_warner_nocontext(packWARN(WARN_LOCALE),
-                            "Unknown locale category %d; can't set it to %s\n",
-                                                     category, locale);
+                          "Unknown locale category %d%s%s",
+                          category, conditional_warn_text, locale);
 
     /* Return an out-of-bounds value */
     return NOMINAL_LC_ALL_INDEX + 1;
@@ -344,45 +349,15 @@ S_get_category_index(const int category, const char * locale)
 STATIC const char *
 S_category_name(const int category)
 {
-    unsigned int i;
-
-#ifdef LC_ALL
+    unsigned int index;
 
-    if (category == LC_ALL) {
-        return "LC_ALL";
-    }
-
-#endif
+    index = get_category_index(category, NULL);
 
-    for (i = 0; i < NOMINAL_LC_ALL_INDEX; i++) {
-        if (category == categories[i]) {
-            return category_names[i];
-        }
+    if (index <= NOMINAL_LC_ALL_INDEX) {
+        return category_names[index];
     }
 
-    {
-        const char suffix[] = " (unknown)";
-        int temp = category;
-        Size_t length = sizeof(suffix) + 1;
-        char * unknown;
-        dTHX;
-
-        if (temp < 0) {
-            length++;
-            temp = - temp;
-        }
-
-        /* Calculate the number of digits */
-        while (temp >= 10) {
-            temp /= 10;
-            length++;
-        }
-
-        Newx(unknown, length, char);
-        my_snprintf(unknown, length, "%d%s", category, suffix);
-        SAVEFREEPV(unknown);
-        return unknown;
-    }
+    return Perl_form_nocontext("%d (unknown)", category);
 }
 
 #endif /* ifdef USE_LOCALE */

pod/perldelta.pod

@@ -764,15 +764,11 @@ diagnostic messages, see L<perldiag>.
 
 =item *
 
-L<Bareword filehandle "%s" not allowed under 'no feature "bareword_filehandles"'|perldiag/"Bareword filehandle "%s" not allowed under 'no feature "bareword_filehandles"'">
+L<Unknown locale category %d|perldiag/"Unknown locale category %d">
 
-This accompanies the new L<bareword_filehandles|feature/"The 'bareword_filehandles' feature."> feature.
-
-=item *
-
-L<Multidimensional hash lookup is disabled|perldiag/"Multidimensional hash lookup is disabled">
-
-This accompanies the new L<multidimensional|feature/"The 'multidimensional' feature"> feature.
+This is a shortened form of an already existing diagnostic, for use when
+there is no new locale being switched to.  The previous diagnostic was
+misleading in such circumstances.
 
 =back
 

pod/perldiag.pod

@@ -6804,6 +6804,8 @@ compiler does not recognize.  Check your spelling.
 (P) Perl was about to print an error message in C<$@>, but the C<$@> variable
 did not exist, even after an attempt to create it.
 
+=item Unknown locale category %d
+
 =item Unknown locale category %d; can't set it to %s
 
 (W locale) You used a locale category that perl doesn't recognize, so it