Add Solaris specific declararions for dlfcn.

Add definitions to `core.sys.posix.dlfcn` and add new `modul core.sys.solaris.dlfcn`.
I added a link to the repository of the Illumos project. I didn't found an official repository at Oracle.

I did not add the missing types because they require `auxv_t` and `mmapobj_result_t` which are declared in other include files. Somewhere I had to stop...