Big rename fest in makefile.shared: link_a / link_o -> link_shlib / link_dso

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: Andy Polyakov <appro@openssl.org>