Review: Remove long_u: memfile: Cleanup: Others.

memfile_defs.h:
- Inline struct definitions in typedefs.
- Move memfile_T definition to this file (weirdly, was in buffer_defs.h).

memfile.c:
- Use C99 style variable declarations. This is, move variable declarations as
  near to first-usage point as possible).
- Modernize old-style function declarations.
- Fix indent at some places (some multiline expressions and the like).

src/nvim/buffer_defs.h

@@ -78,7 +78,6 @@ typedef struct wininfo_S wininfo_T;
 typedef struct frame_S frame_T;
 typedef int scid_T;                     /* script ID */
 typedef struct file_buffer buf_T;       /* forward declaration */
-typedef struct memfile memfile_T;
 
 // for struct memline (it needs memfile_T)
 #include "nvim/memline_defs.h"