Fix multiple compilation errors and memory leaks.

- Rename malloc_message() because it was causing a compilation error on
  FreeBSD.
- Use correct type as a return from gzdopen().
- Memory allocation, header file includes, etc.

src/maildir.c

@@ -122,7 +122,7 @@ maildir_read_message (maildir_t *mdp)
   static FILE *fp;
   static int s;
 
-  message = malloc_message ();
+  message = allocate_message ();
 
   for(;;)
     {