various: use "-" instead of "n/a" in tables

various: use "-" instead of "n/a" in tables

In the context of a table, both would be generally understood to have the same
meaning. "n/a" is a strange beast. It was useful when tables were produced on
the typewriter with "---------" used to separate rows. It is visually more
pleasing to use "-", and there is no risk of it being mistaken for a row
separator.