ECL fixes

- cffi-ecl fixes:
   - push cffi-features:unix on darwin too.
   - use si:load-foreign-module instead of ffi:load-foreign-library
     on DFFI platforms.
   - use convert-external-name in foreign-symbol-pointer.
- wrap defcvar's define-foreign-symbol around eval-when.
- simplify default-library-suffix
- cffi-tests:
   - load libtest.o on ECL platforms without DFFI.
   - conditionalize the tests with 127 arguments based on the
     value of lambda-parameters-limit.