Changes to building dynamically loadable modules

The attached patch implements a few changes to how shared libraries and
dynamically loadable modules are built. Foremost, it creates a solid
distinction between these two types of targets based on what had already been
implemented and duplicated in ad hoc ways before. Specifically,

- Dynamically loadable modules no longer get a soname. The numbers previously
set in the makefiles were dummy numbers anyway, and the presence of a soname
upset a few packaging tools, so it is nicer not to have one.

- The cumbersome detour taken on installation (build a libfoo.so.0.0.0 and
then override the rule to install foo.so instead) is removed.

- Lots of duplicated code simplified.