Made sql_log_slow a session variable

mysqldump:
- Added --log-queries to allow one to disable logging for the dump

sql/log_event.cc:
- Removed setting of enable_slow_log as it's not required anymore.

sql/sql_parse.cc:
- Set enable_slow_log to value of thd->variables.sql_log_slow as this will speed up tests if slow log is disabled.
- opt_log_slow_admin_statements can now only disable slow log, not enable it.

sql/sql_explain.cc:
- Minor cleanup

Other things:
- Added sql_log_slow to system variables.
- Changed opt_slow_log to global_system_variables.sql_log_slow in all files
- Updated tests to reflect changes

sql/sql_explain.cc

@@ -1006,8 +1006,10 @@ int Explain_insert::print_explain(Explain_query *query,
 
 void delete_explain_query(LEX *lex)
 {
+  DBUG_ENTER("delete_explain_query");
   delete lex->explain;
   lex->explain= NULL;
+  DBUG_VOID_RETURN;
 }