`readFromHead`/`writeToTail` should propagate the `Int` returned from the lambda

[JakeWharton]

I'm trying to implement RawSource.readAtMostTo using UnsafeBufferOperations.writeToTail.

I want to do

	override fun readAtMostTo(sink: Buffer, byteCount: Long): Long {
		return UnsafeBufferOperations.writeToTail(sink, 1) { bytes, startIndexInclusive, endIndexExclusive ->
			val read: Int = TODO()
			read
		}.toLong()
	}

but writeToTail doesn't give me the Int which I return from the lambda at the callsite in order to do this.

[JakeWharton]

Expanding this to readFromHead since you can end up reading fewer than requested with things like POSIX write.

[fzhinkin]

There are a few JVM-specific extensions allowing to read/write from/to java.nio.ByteBuffer. It might be worth updating those functions too.