MDEV-16026: Forbid global system_versioning_asof in non-default time zone

* store `system_versioning_asof` in unix time;
* both session and global vars are processed in session timezone;
* setting `default` does not copy global variable anymore. Instead, it sets
  system_time to SYSTEM_TIME_UNSPECIFIED, which means that no 'AS OF' time
  is applied and `now()` can be assumed

As a regression, we cannot assign values below 1970 (UTC) anymore

MDEV-16481: set global system_versioning_asof=sf() crashes in specific case

* sys_vars.h: add `MYSQL_TIME` field to `set_var::save_result`
* sys_vars.ic: get rid of calling `var->value->get_date()` from
 `Sys_var_vers_asof::update()`
* versioning.sysvars: add test; remove double warning

refactor Sys_var_vers_asof

* inherit from sys_var rather than Sys_var_enum
* remove junk "DEFAULT" keyword. There is DEFAULT in SQL grammar for it.
* make all conversions in check() to avoid possible errors
* avoid double var->value evaluation, which could
  consequence in undefined behavior