Bugs fixed:
- Added automatic detection of unsigned arguments to COLUMN_CREATE()
- If string length is not know for COLUMN_GET() use MAX_DYNAMIC_COLUMN_LENGTH instead of MAX_FIELD_BLOBLENGTH
- null_value flag was not propery reset for COLUMN_LIST() and COLUMN_CREATE() which could lead to crashes later:
- lp:778905 Assertion `value->year <= 9999' failed in dynamic_column_date_store
- lp:778912 Assertion `field_pos < field_count' failed in Protocol_text::store in maria-5.3-mwl34

include/ma_dyncol.h:
  Added define for max dynamic column length.
mysql-test/r/cast.result:
  Added test of cast big unsigned int to signed (this test case was missing)
mysql-test/r/dyncol.result:
  Added tests from reported bugs
  Added testing of automatic store of signed/unsigned integers
mysql-test/t/cast.test:
  Added test of cast big unsigned int to signed (this test case was missing)
mysql-test/t/dyncol.test:
  Added tests from reported bugs
  Added testing of automatic store of signed/unsigned integers
sql/item.cc:
  Added assert to catch cases where null_value is not set properly
sql/item_strfunc.cc:
  Added automatic detection of unsigned arguments to COLUMN_CREATE()
  COLUMN_GET() returned wrong value for illegal strings which lead to assert later
  null_value flag was not propery reset for COLUMN_LIST() and COLUMN_CREATE() which could lead to crashes later.
sql/item_strfunc.h:
  If string length is not know for COLUMN_GET() use MAX_DYNAMIC_COLUMN_LENGTH