treewide: use SANE_CURRENT_* macros consistently

Backends use a mixture of SANE_CURRENT_{MAJOR | MINOR} and V_{MAJOR | MINOR} with all kind of permutations. I was confused by this and one comment in pieusb.c tells me I was not alone. Some items in old changelogs suggest to use the SANE_CURRENT_ macros in backends, so let's switch to do that with the exception of net.c and dll.c. Done with:

$ find backend -name '*.[ch]' | xargs sed -i '/nearly every/ ! { s/<V_M/SANE_CURRENT_M/g }' $ git checkout backend/net.c backend/dll.c

And manually removing the comment from pieusb.c. Everything still builds fine.