MDEV-19629 post-merge fixes

* it isn't "pfs" function, don't call it Item_func_pfs,
  don't use item_pfsfunc.*
* tests don't depend on performance schema, put in the main suite
* inherit from Item_str_ascii_func
* use connection collation, not utf8mb3_general_ci
* set result length in fix_length_and_dec
* do not set maybe_null
* use my_snprintf() where possible
* don't set m_value.ptr on every invocation
* update sys schema to use the format_pico_time()
* len must be size_t (compilation error on Windows)
* the correct function name for double->double is fabs()
* drop volatile hack

scripts/sys_schema/functions/format_time.sql

@@ -26,7 +26,9 @@ CREATE DEFINER='mariadb.sys'@'localhost' FUNCTION format_time (
     COMMENT '
              Description
              -----------
-
+             
+             OBSOLETE. USE BUILT-IN format_pico_time() INSTEAD
+             
              Takes a raw picoseconds value, and converts it to a human readable form.
              
              Picoseconds are the precision that all latency values are printed in