Bug #23667 "CREATE TABLE LIKE is not isolated from alteration
             by other connections"
 Bug #18950 "CREATE TABLE LIKE does not obtain LOCK_open"
As well as:
 Bug #25578 "CREATE TABLE LIKE does not require any privileges
             on source table".

The first and the second bugs resulted in various errors and wrong
binary log order when one tried to execute concurrently CREATE TABLE LIKE
statement and DDL statements on source table or DML/DDL statements on its
target table.

The problem was caused by incomplete protection/table-locking against
concurrent statements implemented in mysql_create_like_table() routine.
We solve it by simply implementing such protection in proper way (see
comment for sql_table.cc for details).

The third bug allowed user who didn't have any privileges on table create
its copy and therefore circumvent privilege check for SHOW CREATE TABLE.

This patch solves this problem by adding privilege check, which was missing.

Finally it also removes some duplicated code from mysql_create_like_table().

Note that, altough tests covering concurrency-related aspects of CREATE TABLE
LIKE behaviour will only be introduced in 5.1, they were run manually for
this patch as well.