Add class Sys_var_charptr_fscs

Changes:

• Remove sys_var::is_os_charset
• Remove enum charset_enum:

  enum charset_enum {IN_SYSTEM_CHARSET, IN_FS_CHARSET};

• Remove the charset_enum parameter from Sys_var_charptr* constructors.
• Change sys_var::charset() to be virtual.
• Add a new class Sys_var_charptr_fscs deriving from Sys_var_charptr, to handle system variables with character_set_filesystem.
• Fix variables that were marked as IN_FS_CHARSET to be of the Sys_var_charptr_fscs data type.