FileStream: Approximate the file offset better when appending

Instead of starting at zero, start at the current file size. This offset
is stored in FileStream. It is only an approximation because concurrent
writes could advance the file without FileStream's knowledge.

Add a test that shows that the offset is approximated as expected and that
appending to an existing file works, too.

This is also a regression test which shows that appending to an existing
file works as expected. See vibe-d/eventcore#115.

source/vibe/core/file.d

@@ -438,6 +438,9 @@ struct FileStream {
 		m_ctx.mode = mode;
 		m_ctx.size = eventDriver.files.getSize(fd);
 		m_ctx.driver = () @trusted { return cast(shared)eventDriver; } ();
+
+		if (mode == FileMode.append)
+			m_ctx.ptr = m_ctx.size;
 	}
 
 	this(this)
@@ -787,3 +790,23 @@ version (Windows) {} else unittest {
 	testCreate(".test_foo/", ".test_foo", true);
 	test("/", "", false);
 }
+
+unittest {
+	auto name = "toAppend.txt";
+	scope(exit) removeFile(name);
+
+	{
+		auto handle = openFile(name, FileMode.createTrunc);
+		handle.write("create,");
+		assert(handle.tell() == "create,".length);
+		handle.close();
+	}
+	{
+		auto handle = openFile(name, FileMode.append);
+		handle.write(" then append");
+		assert(handle.tell() == "create, then append".length);
+		handle.close();
+	}
+
+	assert(readFile(name) == "create, then append");
+}