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.
implementation is contained in the application, and the capability
string building part should really be part of the engine library.
This is therefore an experimental hack, and will be changed in the
near future.
