RFFI: abstract native package init support

com.oracle.truffle.r.native/fficall/jni/src/register.c

...oracle/truffle/r/runtime/ffi/generic/Generic_PkgInit.java

...rc/com/oracle/truffle/r/runtime/ffi/jnr/JNI_CallRFFI.java

...src/com/oracle/truffle/r/runtime/ffi/jnr/JNI_PkgInit.java

...com/oracle/truffle/r/runtime/ffi/jnr/JNR_RFFIFactory.java

...e.r.runtime/src/com/oracle/truffle/r/runtime/ffi/DLL.java

...ime/src/com/oracle/truffle/r/runtime/ffi/DLL_PkgInit.java

...ime/src/com/oracle/truffle/r/runtime/ffi/PkgInitRFFI.java

....r.runtime/src/com/oracle/truffle/r/runtime/ffi/RFFI.java

...ime/src/com/oracle/truffle/r/runtime/ffi/RFFIFactory.java