[GR-2798] Various small fixes and cleanups in FFI.

...oracle/truffle/r/ffi/impl/common/JavaUpCallsRFFIImpl.java

...cle/truffle/r/ffi/impl/common/TracingUpCallsRFFIImpl.java

...pl/src/com/oracle/truffle/r/ffi/impl/nodes/MiscNodes.java

...com/oracle/truffle/r/ffi/impl/upcalls/StdUpCallsRFFI.java

...rary/src/com/oracle/truffle/r/library/tools/DirChmod.java

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

...ve/fficall/src/truffle_common/Rinternals_truffle_common.h

...cle.truffle.r.native/fficall/src/truffle_nfi/Rinternals.c

...e/src/com/oracle/truffle/r/runtime/interop/Foreign2R.java