macro: insert it in keystroke buffer without discarding latter's contents

Instead of simply overwriting the current contents of the keystroke
buffer with the contents of the macro buffer, insert the latter's
contents at the head of the keystroke buffer.

This allows using {runmacro} in a string bind, and allows typing ahead
over a laggy connection after invoking `runmacro` (normally with M-;).

This fixes https://savannah.gnu.org/bugs/?65991.
Reported-by: Tasos Papastylianou <tpapastylianou@hotmail.com>

Bug exists since version 2.9.4, since string binds were introduced.

src/prototypes.h

@@ -595,8 +595,8 @@ linestruct *line_from_number(ssize_t number);
 void record_macro(void);
 void run_macro(void);
 #endif
-void reserve_space_for(size_t newsize);
 size_t waiting_keycodes(void);
+void put_back(int keycode);
 #ifdef ENABLE_NANORC
 void implant(const char *string);
 #endif

src/winio.c

@@ -123,14 +123,9 @@ void run_macro(void)
 		return;
 	}
 
-	if (macro_length > capacity)
+	for (size_t index = macro_length; index > 0; )
-		reserve_space_for(macro_length);
+		put_back(macro_buffer[--index]);
 
-	for (size_t i = 0; i < macro_length; i++)
-		key_buffer[i] = macro_buffer[i];
-
-	waiting_codes = macro_length;
-	nextcodes = key_buffer;
 	mute_modifiers = TRUE;
 }
 #endif /* !NANO_TINY */