lseek should take a long offset.
Add ftruncate.
Add fcntlInt that takes one int arg and a no-arg version.

fcntl was modified to take int... but because jnr-ffi does not
support variadic invocation those arguments were never passed to
native. I deprecated the old int... version and added new versions
that pass no args or one int arg. This should cover most use cases
and not mislead anyone into thinking varargs should work.
Use posix.write() directly.
Better tests already in SpawnTest.