ffap-rfc-space

M-x ffap doesn't recognise a space in an RFC like "RFC 1234", only say
"RFC-1234".  The entry in `ffap-alist' in Emacs 22 and earlier allows it,
but the guessing in `ffap-string-at-point' never matched it.  The
following spot of code extends both to do that, adding a (thing-at-point
'rfc) as a bonus.

Perhaps changing the `file' regexp in ffap-string-at-point-mode-alist
would be cleaner.  Or perhaps M-x ffap could tie in with thing-at-point a
bit more, and look for things that have "find" handlers.