New definition of WEEK().


Docs/manual.texi:
  Changelog
myisam/mi_cache.c:
  Better logging on error
myisam/mi_open.c:
  Remove usage of my_dup(), as this casued error 127 when reading tables
mysql-test/r/func_time.result:
  More tests for the WEEK/YEARWEEK function
mysql-test/t/func_time.test:
  More tests for the WEEK/YEARWEEK function
sql/time.cc:
  New definition of WEEK