Change the behavior of `tac` when there are no line separators in the
input. Previously, it was appending an extra line separator; this commit
prevents that from happening. The input is now writted directly to
stdout.
Correct the error message produced by `tac` when trying to read from a
directory. Previously if the path 'a' referred to a directory, then
running `tac a` would produce the error message
dir: read error: Invalid argument
after this commit it produces
a: read error: Invalid argument
which matches GNU `tac`.
