Move the handling of dso_scheme to dso_conf.h

Move the handling of dso_scheme to dso_conf.h

The macros resulting from the dso_scheme attribute were defined for
libraries only, but there's a test program that uses the macros as
well.  The easier way is to move the handling of this macro to
crypto/include/internal/dso_conf.h and having the modules that need it
include it.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5733)