.gitignore: Rearrange and make more sensible.

Things like `deb-build' are my local conventions, so leave them to my
common ignore file.  Ignore all `.so' files rather than just the ones we
know about.  Alas, we can't bulk-ignore the generated scripts, but at
least we can mark the rules for fixed files as being absolute pathnames.

Also add an entry for `/debian/debhelper-build-stamp', since that's
turned up.