Added code to adjust the field_length of user variables 
in dependence on the field type.
Aded new constants for numeric field widths.


include/mysql_com.h:
  BUG#4788 - show create table provides incorrect statement.
  Introduced definitions for default field width of numeric types.
  So common values can be used at different places in the code.
mysql-test/r/variables.result:
  BUG#4788 - show create table provides incorrect statement.
  New test results.
mysql-test/t/variables.test:
  BUG#4788 - show create table provides incorrect statement.
  Added a test for the bug.
sql/item_func.cc:
  BUG#4788 - show create table provides incorrect statement.
  Added code to adjust the field_length of user variables 
  in dependence on the field type.
sql/sql_parse.cc:
  BUG#4788 - show create table provides incorrect statement.
  Changed numeric literals to the new constants.