Add metadata validation to ~20 more SQL commands. Make sure that
these commands actually work in ps-protocol, since until now they
were enabled, but not carefully tested.
Fixes the ml003 bug found by Matthias during internal testing of the
patch.

WL#4165 Prepared statements: validation 
WL#4166 Prepared statements: automatic re-prepare
Fixes
Bug#27430 Crash in subquery code when in PS and table DDL changed after PREPARE
Bug#27690 Re-execution of prepared statement after table was replaced with a view crashes
Bug#27420 A combination of PS and view operations cause error + assertion on shutdown

The basic idea of the patch is to keep track of table metadata between
prepared statement prepare and execute. If some table used in the statement
has changed, the prepared statement is re-prepared before execution.

See WL#4165 and WL#4166 contents and comments in the code for details
of the implementation.

Enable back the disabled test case.

into  mockturtle.local:/home/dlenev/src/mysql-5.1-symlink

The problem was that symlink.test failed due to debug assertions or
due to errors emitted for wrong statements (in non-debug builds)
if 6.0 version of server was run with --thread-handling=pool-of-threads
option.

The above problems were caused by that on the one hand code mi_create()
which handles case when error is returned relies on my_errno being set
to appropriate non-0 value, but on the other it has not done this
in case when error was emitted due to discovery that share for table
which we are going to create is already present in the list of open
shares. Running server with pool-of-threads option just exposed this
bug since in this scheduler mode my_errno is reset to 0 when connection
(THD) is reattached to the "real" thread.

This fix ensures that code in mi_create() properly sets my_errno in
the case described above.

Since original reason that caused this bug to be exposed in 6.0 tree
might also under very special conditions lead to incorrect behavior
of earlier versions of server (i.e. to errors emitted for wrong
statements) this patch is also being pushed into 5.1 tree.

No test case is present as this code is already covered by a test
case + pushbuild options combination in 6.0.

This is a code clean up.
Removed redundant (and unused) TABLE_LIST variable intended as an IN-
parameter for setup_order.

into  adventure.(none):/home/thek/Development/cpp/mysql-5.1-runtime

into  adventure.(none):/home/thek/Development/cpp/mysql-5.0-runtime

into  mysql.com:/Users/davi/mysql/mysql-5.1-runtime

general_log_file/slow_query_log_file.

The problem was that log file path was rejected if directory
path was empty. The fix is to reject log file path only if it
is entirely empty.

We have "set" variables, which can accept empty values
(like sql_mode), and which can not (like log_output). The problem
was that the code does not distinguish them and allow empty
values for every set variable.

The fix is to introduce an attribute of a set variable telling
whether it can accept empty values.

into  quad.opbmk:/mnt/raid/alik/MySQL/devel/5.1-rt-merged

into  mysql.com:/misc/mysql/mysql-5.1-opt

into  mysql.com:/misc/mysql/mysql-5.0-opt

into  mysql.com:/misc/mysql/mysql-5.1-opt

into  host.loc:/home/uchum/work/5.1-opt

into  host.loc:/home/uchum/work/5.0-opt

into  trift2.:/MySQL/M51/push-5.1

into  trift2.:/MySQL/M51/push-5.1

into  trift2.:/MySQL/M50/push-5.0

into  trift2.:/MySQL/M51/push-5.1

into  host.loc:/home/uchum/work/5.1-opt

into  pippilotta.erinye.com:/shared/home/df/mysql/build/mysql-5.1-build

into  host.loc:/home/uchum/work/5.0-opt

into  host.loc:/home/uchum/work/5.1-opt

into  trift2.:/MySQL/M51/push-5.1

into  host.loc:/home/uchum/work/5.0-opt

into  trift2.:/MySQL/M51/push-5.1

into  trift2.:/MySQL/M50/push-5.0

into  quad.opbmk:/mnt/raid/alik/MySQL/devel/5.1-rt-merged

into  rhel5-ia64-a.mysql.com:/data0/tsmith/build/51