Delete structs.h by spliting it and moving code to other headers

Here's the list of squashed commits (for more info, see PR #378).

 - Define guicolor_T as a typedef in syntax.h
 - Move a big chunk of code from structs.h to buffer_defs.h
 - Move aco_save_T from structs.h to fileio.h
 - Move option_table_T from structs.h to hardcopy.h

	Aditionally:

	 - Move the printer_opts global to hardcopy.c
	 - Delete structs.h. Include buffer_defs.h where structs.h was included
	   before.
	 - Add header guards to option_defs.h

 - Put mark types and constants in new mark_defs.h
 - Move undo structs to undo_defs.h
 - Move memfile structs to new memfile_defs.h
 - Move expand_T and cmdmod_T to ex_cmds_defs.h
 - Move memline_T to memline_defs.h
 - Move many defs and types to ex_eval.h
 - Move syntax related types to syntax_defs.h
 - Move struct memfile to memfile_defs.h
 - struct buffblock and struct buffheader moved back to buffer_defs.h
 - Move some datatypes to hashtab.h and eval_defs.h
 - Move the buffer_defs.h include and TODOs for remaining unrelated types in buffer_defs.h

src/fileio.h

@@ -1,5 +1,21 @@
 #ifndef NEOVIM_FILEIO_H
 #define NEOVIM_FILEIO_H
+
+#include "buffer_defs.h"
+
+/*
+ * Struct to save values in before executing autocommands for a buffer that is
+ * not the current buffer.  Without FEAT_AUTOCMD only "curbuf" is remembered.
+ */
+typedef struct {
+  buf_T       *save_curbuf;     /* saved curbuf */
+  int use_aucmd_win;            /* using aucmd_win */
+  win_T       *save_curwin;     /* saved curwin */
+  win_T       *new_curwin;      /* new curwin */
+  buf_T       *new_curbuf;      /* new curbuf */
+  char_u      *globaldir;       /* saved value of globaldir */
+} aco_save_T;
+
 /* fileio.c */
 void filemess(buf_T *buf, char_u *name, char_u *s, int attr);
 int readfile(char_u *fname, char_u *sfname, linenr_T from,