sane-backends 1.0.20, finally.

Almost two months after sane-backends-1.0.20 appeared upstream I finally managed to complete the first ebuild attempt.

It took me so long because

• there were a few internal changes with the new version and I wanted to check if the ebuild is still OK
• I made SANE_BACKENDS a USE_EXPAND variable now. So you can see which backends are available and enabled without reading the ebuild or the SANE homepage. Well, and it gets a lot harder to attempt building non-existant backends as some people tried in the past for understandable reasons.
• I can't complain about too much spare time and I hardly ever get to do anything that takes more than an hour or two, and I was new to the USE_EXPAND business.

Some frontends (including sane-frontends xscanimage) fail(ed) to compile with this version because a hardly ever needed flag named SANE_CAP_ALWAYS_SETTABLE which was not part of the official API (though in sane.h) has been removed. If you run into problems because of this please let the maintainer of the frontend know, it is not really sane-backends' fault.