MDEV-21956 Add class Sys_var_charptr_fscs

Simplifying definitions on Sys_var_charptr*:

- Removing sys_var::is_os_charset
- Adding a new class Sys_var_charptr_fscs, to handle system
  variables with character_set_filesystem.

sql/set_var.cc

@@ -154,8 +154,7 @@ sys_var::sys_var(sys_var_chain *chain, const char *name_arg,
   next(0), binlog_status(binlog_status_arg), value_origin(COMPILE_TIME),
   flags(flags_arg), show_val_type(show_val_type_arg),
   guard(lock), offset(off), on_check(on_check_func), on_update(on_update_func),
-  deprecation_substitute(substitute),
-  is_os_charset(FALSE)
+  deprecation_substitute(substitute)
 {
   /*
     There is a limitation in handle_options() related to short options:
@@ -509,12 +508,6 @@ bool throw_bounds_warning(THD *thd, const char *name, bool fixed, double v)
   return false;
 }
 
-CHARSET_INFO *sys_var::charset(THD *thd)
-{
-  return is_os_charset ? thd->variables.character_set_filesystem :
-    system_charset_info;
-}
-
 
 typedef struct old_names_map_st
 {

sql/set_var.h

@@ -90,7 +90,6 @@ class sys_var: protected Value_source // for double_from_string_with_check
   on_check_function on_check;
   on_update_function on_update;
   const char *const deprecation_substitute;
-  bool is_os_charset; ///< true if the value is in character_set_filesystem
 
 public:
   sys_var(sys_var_chain *chain, const char *name_arg, const char *comment,
@@ -130,7 +129,10 @@ class sys_var: protected Value_source // for double_from_string_with_check
 
   SHOW_TYPE show_type() { return show_val_type; }
   int scope() const { return flags & SCOPE_MASK; }
-  CHARSET_INFO *charset(THD *thd);
+  virtual CHARSET_INFO *charset(THD *thd) const
+  {
+    return system_charset_info;
+  }
   bool is_readonly() const { return flags & READONLY; }
   /**
     the following is only true for keycache variables,