Big rename fest of engine DSO names, from libFOO.so to FOO.so

The engine DSOs were named as if they were shared libraries, and could
end up having all sorts of fancy names:

  Cygwin: cygFOO.dll
  Mingw:  FOOeay32.dll
  Unix:   libFOO.so / libFOO.sl / libFOO.dylib / ...

This may be confusing, since they look like libraries one should link
with at link time, when they're just DSOs.

It's therefore time to rename them, and do it consistently on all
platforms:

  Cygwin & Mingw: FOO.dll
  Unix:           FOO.{so,sl,dylib,...}

Interestingly enough, the MSVC and VMS builds always did it this way.

Reviewed-by: Andy Polyakov <appro@openssl.org>