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_plugin.h

@@ -16,6 +16,22 @@
 #ifndef _sql_plugin_h
 #define _sql_plugin_h
 
+/*
+  the following #define adds server-only members to enum_mysql_show_type,
+  that is defined in plugin.h
+*/
+#define SHOW_always_last SHOW_KEY_CACHE_LONG, \
+            SHOW_KEY_CACHE_LONGLONG, SHOW_LONG_STATUS, SHOW_DOUBLE_STATUS, \
+            SHOW_HAVE, SHOW_MY_BOOL, SHOW_HA_ROWS, SHOW_SYS, \
+            SHOW_LONG_NOFLUSH, SHOW_LONGLONG_STATUS, SHOW_LEX_STRING
+#include <mysql/plugin.h>
+#undef SHOW_always_last
+
+#include "m_string.h"                       /* LEX_STRING */
+#include "my_alloc.h"                       /* MEM_ROOT */
+
+#include "my_sys.h"                         /* CALLER_INFO_PROTO */
+
 class sys_var;
 enum SHOW_COMP_OPTION { SHOW_OPTION_YES, SHOW_OPTION_NO, SHOW_OPTION_DISABLED};