The '@' symbol can not be used in the host name according to rfc952.
The fix:
added function check_host_name(LEX_STRING *str)
which checks that all symbols in host name string are valid and
host name length is not more than max host name length
(just moved check_string_length() function from the parser into check_host_name()).


mysql-test/r/create.result:
  test result
mysql-test/t/create.test:
  test case
sql/mysql_priv.h:
  added function check_host_name(LEX_STRING *str)
sql/sql_parse.cc:
  added function check_host_name(LEX_STRING *str)
  which checks that all symbols in host name string are valid and
  host name length is not more than max host name length(HOSTNAME_LENGTH).
sql/sql_yacc.yy:
  using newly added function check_host_name()