drop CVS keywords

OpenBSD is using CVS to manage it's port repository, from which packages are built. Sometimes OpenBSD specific patches on top of upstream's code are needed. Very rarely modification of source file is needed, where upstream's CVS keyword is located. Those CVS keywords end up in the diff content, as part of context lines of the diff (-u option to diff(1) command). Those diffs are then committed into OpenBSD ports tree, which is CVS, so CVS expands those keywords and that mangles the diff. Some manual workarounds are then needed to generate valid patch, but avoid CVS keyword expansion. Unfortunately sane-backends is currently one of those affected ports. As sane-backends is managed by git, those CVS keyword are, I believe, relic of the past. To make life easier for OpenBSD porters, I propose the diff to drop (almost all) CVS Id keywords from the sane-backends source tree.