Do not use `fcopyfile` if appending to non-empty file [Bug #18388]

	`fcopyfile` appends `src` to `to` and then truncates `to` to it's
	original size.
	---
	 io.c                 |  7 +++++++
	 test/ruby/test_io.rb | 12 ++++++++++++
	 2 files changed, 19 insertions(+)