The function ‘vc-default-dir-extra-headers’ is implemented as a stub
function that prints in the vc-dir buffer a rather annoying and
unhelpful message advicing that the implementer of a VC backend should
create some backend specific headers and that doing this is easy.
vc-rcs doesn't implement any (and I doubt it needs), and thus whenever I
use vc-dir on an RCS repository, this message is shown.  Attached is a
patch that disables this behaviour by making the said function return
nil instead (the function as it is includes a comment that says to do so
before the ‘release’).