incomplete in 5.0 (and review fixes).

When in 5.0.13 I introduced class Prepared_statement and methods
::prepare and ::execute, general logging was left out of this class.
This was good for stored procedures, since in stored procedures
we do not log sub-statements, but introduced a regression in case of SQL
syntax for prepared statements, as previously we would log the actual
statements to the log, and after the change we would log only
COM_QUERY text.

Restore the old behavior, but still suppress logging if inside a stored 
procedure.

Based on a community contributed patch from Vladimir Shebordaev.

No test case since we do not have a mechanism to test output
of the general log.