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/sql_error.h

@@ -19,6 +19,7 @@
 
 #include "sql_list.h" /* Sql_alloc, MEM_ROOT */
 #include "m_string.h" /* LEX_STRING */
+#include "sql_string.h"                        /* String */
 #include "mysql_com.h" /* MYSQL_ERRMSG_SIZE */
 
 class THD;