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/hardcopy.h

@@ -32,6 +32,42 @@ typedef struct {
   char_u      *arguments;
 } prt_settings_T;
 
+/*
+ * Generic option table item, only used for printer at the moment.
+ */
+typedef struct {
+  const char  *name;
+  int hasnum;
+  long number;
+  char_u      *string;          /* points into option string */
+  int strlen;
+  int present;
+} option_table_T;
+
+#define OPT_PRINT_TOP          0
+#define OPT_PRINT_BOT          1
+#define OPT_PRINT_LEFT         2
+#define OPT_PRINT_RIGHT        3
+#define OPT_PRINT_HEADERHEIGHT 4
+#define OPT_PRINT_SYNTAX       5
+#define OPT_PRINT_NUMBER       6
+#define OPT_PRINT_WRAP         7
+#define OPT_PRINT_DUPLEX       8
+#define OPT_PRINT_PORTRAIT     9
+#define OPT_PRINT_PAPER        10
+#define OPT_PRINT_COLLATE      11
+#define OPT_PRINT_JOBSPLIT     12
+#define OPT_PRINT_FORMFEED     13
+#define OPT_PRINT_NUM_OPTIONS  14
+
+/* For prt_get_unit(). */
+#define PRT_UNIT_NONE  -1
+#define PRT_UNIT_PERC  0
+#define PRT_UNIT_INCH  1
+#define PRT_UNIT_MM    2
+#define PRT_UNIT_POINT 3
+#define PRT_UNIT_NAMES {"pc", "in", "mm", "pt"}
+
 #define PRINT_NUMBER_WIDTH 8
 
 char_u *parse_printoptions(void);