WIP: Fix #3316: javalib FileChannel APPEND relative writes now match JVM

[LeeTibbert]

Fix #3316

The Java 8 documentation for SeekableByteChannel$position(pos) says:

Setting the channel's position is not recommended when connected to an entity, 
typically a file, that is opened with the APPEND option. 

However, the JVM implementation allows it and APPEND relative writes happen at the expected place: End Of File.

As of this PR, Scala Native implements the same proscribed but not prohibited behavior.

The design center for this PR was correctness. There is no benchmark data to show it, but
both the simple and APPEND channel write(ByteBuffer) paths should be quicker
and more efficient since a number of unnecessary file movement (lseek) calls were
eliminated.

This PR also adds some improved error reporting on not-Windows and a more
direct way, and hopefully better, way of determining file size on not-Windows.

Note:

The JVM documentation states that FileChannels are thread-safe. This implementation, and probably most other,
javalib classes outside of java.util.concurrent are not.

~~ editorial opinion about having more than one thread using a FileChannel redacted ~~

Multiple threads on the same FileChannel may have undesired results.

[LeeTibbert]

Moving to Work-In-Progress sideline while I study why this PR fails across the board on CI
and works on both my macOS & Linux systems.

Issue seems to be with checking that the position is indeed 0 after an error-free position(0).

[LeeTibbert]

This WIP PR has served it purpose well. It lead to a number (5? 10?) of now merged
PRs. The most relevant is PR #3368, which solves the problem which motivated this PR.