Backport for 5.5

In non debug builds, the statements:
- SHOW PROCEDURE CODE
- SHOW FUNCTION CODE
used to fail with a "syntax error", which is misleading.

These statements have been changed to return the following error for non
debug builds:
ERROR HY000: The 'SHOW PROCEDURE|FUNCTION CODE' feature is disabled; you
need MySQL built with '--with-debug' to have it working

For debug builds (./configure --with-debug), nothing is changed.