WL#5030: Split and remove mysql_priv.h

This patch:

- Moves all definitions from the mysql_priv.h file into
  header files for the component where the variable is
  defined
- Creates header files if the component lacks one
- Eliminates all include directives from mysql_priv.h
- Eliminates all circular include cycles
- Rename time.cc to sql_time.cc
- Rename mysql_priv.h to sql_priv.h

sql/spatial.h

@@ -16,8 +16,12 @@
 #ifndef _spatial_h
 #define _spatial_h
 
+#include "sql_string.h"                         /* String, LEX_STRING */
+
 #ifdef HAVE_SPATIAL
 
+class Gis_read_stream;
+
 const uint SRID_SIZE= 4;
 const uint SIZEOF_STORED_DOUBLE= 8;
 const uint POINT_DATA_SIZE= SIZEOF_STORED_DOUBLE*2;