- dropped routines should not show up in status
- error handling for non-existing routines

+ some cleanup.


mysql-test/r/sp-error.result:
  Test SHOW CREATE PROCEDURE for non-existing procedure.
mysql-test/r/sp.result:
  Additional SHOW FUNCTION/PROCEDURE STATUS calls (make sure they don't show
  after being dropped).
mysql-test/t/sp-error.test:
  Test SHOW CREATE PROCEDURE for non-existing procedure.
mysql-test/t/sp.test:
  Additional SHOW FUNCTION/PROCEDURE STATUS calls (make sure they don't show
  after being dropped).
sql/sp.cc:
  Fixed bug in SHOW ... STATUS after a routine has been dropped,
  and fixed the error return codes (for correct error handling).
  Also some general cleanup.
sql/sp.h:
  Fixed prefix for external functions (should be sp_, not db_).
sql/sql_parse.cc:
  Fixed error handling in SHOW CREATE PROCEDURE/FUNCTION.