Deprecate PERLIO_K_DUMMY as it doesn't do anything

In fact, it hasn't done anything for like a decade

Author: Leont

perlio.c

@@ -1024,7 +1024,7 @@ PERLIO_FUNCS_DECL(PerlIO_remove) = {
     sizeof(PerlIO_funcs),
     "pop",
     0,
-    PERLIO_K_DUMMY | PERLIO_K_UTF8,
+    PERLIO_K_UTF8,
     PerlIOPop_pushed,
     NULL,
     PerlIOBase_open,
@@ -2243,7 +2243,7 @@ PERLIO_FUNCS_DECL(PerlIO_utf8) = {
     sizeof(PerlIO_funcs),
     "utf8",
     0,
-    PERLIO_K_DUMMY | PERLIO_K_UTF8 | PERLIO_K_MULTIARG,
+    PERLIO_K_UTF8 | PERLIO_K_MULTIARG,
     PerlIOUtf8_pushed,
     NULL,
     PerlIOBase_open,
@@ -2274,7 +2274,7 @@ PERLIO_FUNCS_DECL(PerlIO_byte) = {
     sizeof(PerlIO_funcs),
     "bytes",
     0,
-    PERLIO_K_DUMMY | PERLIO_K_MULTIARG,
+    PERLIO_K_MULTIARG,
     PerlIOUtf8_pushed,
     NULL,
     PerlIOBase_open,
@@ -2305,7 +2305,7 @@ PERLIO_FUNCS_DECL(PerlIO_raw) = {
     sizeof(PerlIO_funcs),
     "raw",
     0,
-    PERLIO_K_DUMMY,
+    0,
     PerlIORaw_pushed,
     PerlIOBase_popped,
     PerlIOBase_open,

perliol.h

@@ -57,7 +57,7 @@ struct _PerlIO_funcs {
 #define PERLIO_K_BUFFERED	0x00000002
 #define PERLIO_K_CANCRLF	0x00000004
 #define PERLIO_K_FASTGETS	0x00000008
-#define PERLIO_K_DUMMY		0x00000010
+#define PERLIO_K_DUMMY		0x00000000 /* DEPRECATED */
 #define PERLIO_K_UTF8		0x00008000
 #define PERLIO_K_DESTRUCT	0x00010000
 #define PERLIO_K_MULTIARG	0x00020000