CLIENT/CONNECTION CHARSET

Problem was caused by init_common_variables() function which uses
--character-set-server value to assign default_charset_info global
variable and 5 charset variables in global_system_variables structure.

Fixed by checking if charset value is acceptable for client-related
variables. is_supported_parser_charset() function used for the check.

Additional check implemented in thd_init_client_charset() - if client
charset is not supported then error is generated.

parse_client_handshake_packet() fixed to use
global_system_variables.character_set_client instead of
default_charset_info when client does not provide its character set