Change the engine library so the application writer has to explicitely

load the "external" built-in engines (those that require DSO).  This
makes linking with libdl or other dso libraries non-mandatory.

Change 'openssl engine' accordingly.

Change the engine header files so some declarations (that differed at
that!) aren't duplicated, and make sure engine_int.h includes
engine.h.  That way, there should be no way of missing the needed
info.