Updated documentation files to reflect MariaDB and not the Maria storage engine or MySQL

Added (rewritten) patch from Percona to get extended statistics in slow.log:
- Added handling of 'set' variables to set_var.cc. Changed sql_mode to use this
- Added extra logging to slow log of 'Thread_id, Schema, Query Cache hit, Rows sent and Rows examined'
- Added optional logging to slow log, through log_slow_verbosity, of query plan statistics
- Added new user variables log_slow_rate_limit, log_slow_verbosity, log_slow_filter
- Added log-slow-file as synonym for 'slow-log-file', as most slow-log variables starts with 'log-slow'
- Added log-slow-time as synonym for long-query-time
Some trivial MyISAM optimizations:
- In prepare for drop, flush key blocks
- Don't call mi_lock_database if my_disable_locking is used

KNOWN_BUGS.txt:
  Updated file to reflect MariaDB and not the Maria storage engine
README:
  Updated file to reflect MariaDB
mysql-test/r/log_slow.result:
  Test new options for slow query log
mysql-test/r/variables.result:
  Updated result (old version cut of things at 79 characters)
mysql-test/t/log_slow.test:
  Test new options for slow query log
sql/Makefile.am:
  Added log_slow.h
sql/event_data_objects.cc:
  Removed not needed test for enable_slow_log (is done when the flag is tested elsewhere)
sql/events.cc:
  Use the general make_set() function instead of 'symbolic_mode_representation'
sql/filesort.cc:
  Added status for used query plans
sql/log.cc:
  Reset counters if no query_length (from Percona's patch; Not sure if needed, but can do no harm)
  Added extra logging to slow log of 'Thread_id, Schema, Query Cache hit, Rows sent and Rows examined'
  Added optional logging to slow log, through log_slow_verbosity, of query plan statistics
  Fixed wrong test of error condition
sql/log_slow.h:
  Defines and variables for log_slow_verbosity and log_slow_filter
sql/mysql_priv.h:
  Include log_slow.h
sql/mysqld.cc:
  Added new user variables log_slow_rate_limit, log_slow_verbosity, log_slow_filter
  Added log-slow-file as synonym for 'slow-log-file', as most slow-log variables starts with 'log-slow'
  Added log-slow-time as synonym for long-query-time
  Added note that one should use log-slow-filter instead of log-slow-admin-statements
  Updated comment from 'slow_query_log_file'
sql/set_var.cc:
  Added long_slow_time as synonym for long_query_time
  Added new user variables log_slow_rate_limit, log_slow_verbosity, log_slow_filter
  dded handling of 'set' variables to set_var.cc. Changed sql_mode to use this
sql/set_var.h:
  - Added handling of 'set' variables. Changed sql_mode to use this
sql/slave.cc:
  Use global filter also for slaves
sql/sp_head.cc:
  Simplify saving of general_slow_log state
  Use the general make_set() function instead of 'symbolic_mode_representation'
sql/sql_cache.cc:
  Added status for used query plans
sql/sql_class.cc:
  Remember/restore query_plan_flags over complex statements
sql/sql_class.h:
  Added variables to handle extended slow log statistics
sql/sql_parse.cc:
  Added status for used query plans
  Added test for filtering slow_query_log
sql/sql_select.cc:
  Added status for used query plans
sql/sql_show.cc:
  Use the general make_set() function instead of 'symbolic_mode_representation'
sql/strfunc.cc:
  Report first error (not last) if something is wrong in a set
  Removed compiler warning
storage/myisam/mi_extra.c:
  In prepare for drop, flush key blocks (speed optimization)
storage/myisam/mi_locking.c:
  Don't call mi_lock_database if my_disable_locking is used (speed optimization)

sql/filesort.cc

@@ -188,6 +188,7 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   {
     status_var_increment(thd->status_var.filesort_scan_count);
   }
+  thd->query_plan_flags|= QPLAN_FILESORT;
 #ifdef CAN_TRUST_RANGE
   if (select && select->quick && select->quick->records > 0L)
   {
@@ -253,6 +254,7 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   }
   else
   {
+    thd->query_plan_flags|= QPLAN_FILESORT_DISK;
     if (table_sort.buffpek && table_sort.buffpek_len < maxbuffer)
     {
       x_free(table_sort.buffpek);
@@ -1199,6 +1201,7 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
   DBUG_ENTER("merge_buffers");
 
   status_var_increment(current_thd->status_var.filesort_merge_passes);
+  current_thd->query_plan_fsort_passes++;
   if (param->not_killable)
   {
     killed= &not_killable;