By default MyISAM overwrites .MYD and .MYI files no 
DATA DIRECTORY option is used. This can lead to two tables
using the same .MYD and .MYI files (that can't be dropped).

To prevent CREATE TABLE from overwriting a file a new option
is introduced : keep_files_on_create
When this is on the CREATE TABLE throws an error if either
the .MYD or .MYI exists for a MyISAM table.
The option is off by default (resulting in compatible behavior).


include/my_base.h:
  Bug #29325: introduce keep_files_on_create
myisam/mi_create.c:
  Bug #29325: introduce keep_files_on_create
mysql-test/r/create.result:
  Bug #29325: test case
mysql-test/t/create.test:
  Bug #29325: test case
sql/ha_myisam.cc:
  Bug #29325: introduce keep_files_on_create
sql/set_var.cc:
  Bug #29325: introduce keep_files_on_create
sql/sql_class.h:
  Bug #29325: introduce keep_files_on_create
sql/sql_table.cc:
  Bug #29325: introduce keep_files_on_create
sql/unireg.cc:
  Bug #29325: introduce keep_files_on_create