basename: fix history

Basename(1) first appeared in the 7th edition. It was not in the 6th
edition, or PWB releases. It's on all the subsequent descendants.

Dirname(1) first appeared in System III, and was later picked up in
4.3-Reno and 8th edition research unix (though was not in 4.1BSD where
the bulk of 8th edition came from). In System III and V8 it was a shell
script, though the BSD version is in C.

Sponsored by: Netflix