db: record which system a particular build was for.

db: record which system a particular build was for.

the biggest side-effect of this change is the database schema must change in a way that
sqlite will not let happen without rebuilding the database. (Hence take this
opportunity to rename git_revision to scm_revision.)

cran2deb essentially behaves as if it cannot see any builds for any system
other than the currently configured one.

Also: note some changes in the documentation. One bit just has a fat warning as
I am feeling lazy. Sorry.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@158 edb9625f-4e0d-4859-8d74-9fd3b1da38cb