Originally, the Makefile.shared targets described what they used as
input for a shared object, be it a shared library or a DSO.  It turned
out, however, that the link_o targets were used exclusively for
engines and the link_a targets were for libcrypto and libssl.

This rename fest turns and indication on the kind of input the targets
get to the intention with using them.
Reviewed-by: NAndy Polyakov <appro@openssl.org>