plan9: support %r format to read errstr

Author: Noam Preil

src/core/pp.c

@@ -794,9 +794,7 @@ static const char *get_fmt_mapping(char c) {
         if (format_mappings[i].c == c)
             return format_mappings[i].mapping;
     }
-#ifndef JANET_PLAN9
     janet_assert(0, "bad format mapping");
-#endif
 }
 
 static const char *scanformat(
@@ -1025,10 +1023,19 @@ void janet_buffer_format(
             char form[MAX_FORMAT], item[MAX_ITEM];
             char width[3], precision[3];
             int nb = 0; /* number of bytes in added item */
+#ifdef JANET_PLAN9
+			if(*strfrmt == 'r'){
+				rerrstr(item, MAX_ITEM);
+				nb = strlen(item);
+			} else
+#endif
             if (++arg >= argc)
                 janet_panic("not enough values for format");
             strfrmt = scanformat(strfrmt, form, width, precision);
             switch (*strfrmt++) {
+#ifdef JANET_PLAN9
+				case 'r': break;
+#endif
                 case 'c': {
                     nb = snprintf(item, MAX_ITEM, form, (int)
                                   janet_getinteger(argv, arg));