fix bison warnings (clash <NONE> != <>)
.gitignore ninja files
MDEV-6107 merge default_tmp_storage_engine

Adapt default_tmp_storage_engine implementation from mysql-5.6
New feature (as compared to 5.6), default_tmp_storage_engine=NULL
means that temporary tables will use default_storage_engine value.
This makes the behavior backward compatible.
MDEV-6137 better help for SET/ENUM sysvars

Auto-generate the allowed list of values for enum/set/flagset options
in --help output. But don't do that when the help text already has them.

Also, remove lists of values from help strings of various options, where
they were simply listed without any additional information.