Add Solaris specific declararions for dlfcn.

Add definitions to `core.sys.posix.dlfcn` and add new `modul core.sys.solaris.dlfcn`.
Fixed, too.

BTW: The files `mak/SRC`, `mak/COPY` and `mak/MANIFEST` do not contain the same set of files. E.g. `core/sys/solaris/execinfo.d` is missing only in `mak/SRC`. Same is true for some `core/sys/linux/*` modules. Is this by design or by error?