Moved all range checks for TIMESTAMP value to my_gmt_sec().
Also fixed check of upper boundary of TIMESTAMP range (which 
also now will catch datetime values which are too small for
TIMESTAMP in case if time_t is unsigned).  


mysql-test/r/timezone.result:
  Added test which checks if TIMESTAMP range is checked 
  correctly (current time zone is honoured and both upper 
  and lower bounds of TIMESTAMP range are checked).
mysql-test/t/timezone.test:
  Added test which checks if TIMESTAMP range is checked 
  correctly (current time zone is honoured and both upper 
  and lower bounds of TIMESTAMP range are checked).
sql/field.cc:
  Check if datetime value is in TIMESTAMP range has moved to 
  my_gmt_sec() function.
sql/mysql_priv.h:
  Added more constants for checking if datetime is in allowed
  range for TIMESTAMP.
sql/time.cc:
  Check if datetime value is in TIMESTAMP range has moved to 
  my_gmt_sec() function. Fixed check of its return value
  to catch overflows in both directions and also overflows in 
  case of unsigned time_t.