DB 2.7.x is not binary compatible with DB 2.4.x

[Mark Kettenis]

I've verified that DB 2.7.5 (the version on the main branch) is not
binary compatible with DB 2.4.14 (the version included with 2.1.x).
Sendmail cannot create the aliases database (/etc/aliases.db)
anymore.  If you compare db.h between those versions (using the CVS
web browser on sourceware) you'll see that a lot of flags have been
renumbered.  In particular the DB_TRUNCATE flag, which is causing my
sendmail troubles.

This means that we should either bump the libdb soname, or use
versioning to create functions that map the old flags onto the new
ones.  I think the latter involves quite some work, and is not
guaranteed to result in total binary compatibility.  So personally, I'm
in favour of bumping the soname.

Mark