Fixed the following compilation errors and test failures:
- maria SE: "stage_waiting_for_a_resource" wasn't declared w/o PFS
- sql_repl.h: PSI_mutex_key is not available in non-PFS builds
- mysqld.cc: pfs_param is not available in non-PFS builds
- mysqld.cc: init_show_explain_psi_keys() is not available in non-PFS builds
- mysqld.cc: call net_before_header_psi, net_after_header_psi even if PFS
  is not available so that thread enters stage_init at proper time. Fixes
  sp-threads and a few tests in funcs_1.
- myisam_file_io.opt: added missing loose prefix

        Added the IF NOT EXISTS option to the CONSTRAINT keyword.

MDEV-5248 Serious incompatibility and data corruption of DATETIME and DATE types due to get_innobase_type_from_mysql_type refactor combined with InnoDB Online DDL

restore old innodb get_innobase_type_from_mysql_type() function,
record all mysql_type->innodb_type mapping
(as generated by mysql-5.6).
add safety code to disable online alter when internal types don't match

storage/innobase/dict/dict0stats.cc:
  revert to 5.6 state

correct bugs in mysql_system_tables_fix.sql.
Update system_mysql_db_fix* tests

Patch by Elena.

Move the table creation to the end of the file, so
mysql.innodb_stats_table has been created and the statement
does not fail.

Apply fix suggested by Igor:
- When eliminate_item_equal() generates pair-wise equalities from a 
  multi-equality,  do generate a "bridge" equality between the first 
  field inside SJM nest and the field that's first in the overall multi-equality.

MDEV-5113 Wrong result (extra row) and valgrind warnings in Item_maxmin_subselect::any_value on 2nd execution of PS with SELECT subquery

When setting Item_func_not_all::test_sum_item or Item_func_not_all::test_sub_item,
reset the other one to NULL - they can never be set both. When a PS is reexecuted,
different executions might be optimized differently and a wrong test_su*_item
might stay set from the previous execution.

- MIN/MAX optimizer does a check whether a "field CMP const" comparison uses a constant 
  that's longer than the field it is compared to.  Make this check only for string columns,
  also compare character lengths, not byte lengths.

- Address input from the mail list: change how EXPLAIN is 
  formatted in the slow query log.

The used_tables attribute must be recalculated for the HAVING condition
if the condition is applied to the rows read from a temporary table.

allocate thd_proc_info string in thd memroot, not on the stack, so that it
won't be overwritten while another thread might be printing it

sort Com_ counters. No simple fix for Binlog_ variables.

        Use sql_field->change parameter as the name of the field.

MDEV-5153: Server crashes in Item_ref::fix_fields on 2nd execution of PS with LEFT JOIN and MERGE view or SELECT SQ

1. Transformation of row IN subquery made the same as single value.
2. replace_where_subcondition() made working on several layers of OR/AND because it called on expression before fix_fields().

(and valgrind warnings)

* move thd userstat initialization to the same function
  that was adding thd userstat to global counters.
* initialize thd->start_bytes_received in THD::init
  (when thd->userstat_running is set)

Singular Item_equal support added.

The problem was that during constant table substitution Item_equal become containing only one constant which was not supported internally.

MDEV-4435 Server crashes in my_strcasecmp_utf8 on ADD KEY IF NOT EXISTS with implicit name when the key exists.
        Use field name as a key name if the key name wasn't specified.

A clean-up: removing the code catching collation incompatibilities
from handler::check_collation_compatibility(), as the collation IDs
are already replaced at this point by TABLE_SHARE::init_from_binary_frm_image.

mysql_upgrade should do --skip-write-binlog by default

by convention query execution time should be 0 if its start_time is 0
(this was lost when fixing MDEV-4578)

port mysqld_safe numa extensions from percona-server: --flush-caches and --numa-interleave

1. use an appropriate errno code
2. put a comma between the errno and the error message text

before running mysql_install_db check for the existence
of $datadir/mysql, not simply $datadir ($datadir might be mounted on
a separate device - exists, but empty)

fix tests where a path was used as a regex.
revert changes to sys_vars.character_sets_dir_basic - we don't run it on windows anyway

quote the path when using it in a regex - the path might contain wildcards (e.g. +)

expand fullhostname inside the string, to have an empty string, not nothing,
when fullhostname is not defined