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/misc.h

@@ -29,7 +29,7 @@ format_t folder_format (const char *name);
 lockmethod_t lock_method (const char *name);
 /* time_t parse_date(char *datestr); */
 char * parse_return_path(char *rpath);
-void * malloc_message (void);
+void * allocate_message (void);
 void postmark_print (message_t *msg);
 void set_default_options (void);
 void get_runtime_options (int *argc, char **argv, struct option *long_options);