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.