Remake the installation of shared libraries in unix-Makefile.tmpl

Instead of having the installation recipe rely on special knowledge,
feed it with information, including what shared library files belong
together.  For Cygwin and Mingw, that's the .dll and its import
library .dll.a.  For Unixen, it's the shared library file name with SO
version and the one without.

Reviewed-by: Andy Polyakov <appro@openssl.org>