Make maximum blob size to be 2**32-1, regardless of word size.

Fix failure of timestamp with size of 2**31-1.  The method of
rounding up to the nearest even number would overflow.


mysql-test/r/type_blob.result:
  2**32-1 is not a special case for timestamp.
  
  Test 2**32-1 and 2**64 as the reliable test points for both 32-
  and 64-bit machines.  I'd like to test 2**32, but that would make 
  tests that vary between architectures.
  
  I'd like to generalize the tests by pulling the max blob size from
  the server, and then "eval"ing N-1, N, and N+1 instead of all these
  literal numbers, but I have not found a way to get UINT_MAX.
mysql-test/t/type_blob.test:
  2**32-1 is not a special case for timestamp.
  
  Test 2**32-1 and 2**64 as the reliable test points for both 32-
  and 64-bit machines.  I'd like to test 2**32, but that would make 
  tests that vary between architectures.
  
  I'd like to generalize the tests by pulling the max blob size from
  the server, and then "eval"ing N-1, N, and N+1 instead of all these
  literal numbers, but I have not found a way to get UINT_MAX.
sql/field.cc:
  Fix a bug where the round-to-even code for TIMESTAMP fields
  failed where the size would overflow the size to zero and then
  fail.
  
  Also, since we silently truncate the size of TIMESTAMP fields, set
  the maximum size we report is allowable to be the largest parsable
  number.
sql/unireg.h:
  Make BLOB size the maximum that the packed value in 
  field_blob::get_length() allows.