lib/fs: Properly handle case insensitive systems (fixes #1787, fixes #2739, fixes #5708)

With this change we emulate a case sensitive filesystem on top of
insensitive filesystems. This means we correctly pick up case-only renames
and throw a case conflict error when there would be multiple files differing
only in case.

This safety check has a small performance hit (about 20% more filesystem
operations when scanning for changes). The new advanced folder option
`caseSensitiveFS` can be used to disable the safety checks, retaining the
previous behavior on systems known to be fully case sensitive.

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs_realcaser_windows.go

@@ -0,0 +1,58 @@
+// Copyright (C) 2020 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build windows
+
+package fs
+
+import (
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+type basicRealCaserWindows struct {
+	uri string
+}
+
+func newBasicRealCaser(fs Filesystem) realCaser {
+	return &basicRealCaserWindows{fs.URI()}
+}
+
+// RealCase returns the correct case for the given name, which is a relative
+// path below root, as it exists on disk.
+func (r *basicRealCaserWindows) realCase(name string) (string, error) {
+	if name == "." {
+		return ".", nil
+	}
+	path := r.uri
+	comps := strings.Split(name, string(PathSeparator))
+	var err error
+	for i, comp := range comps {
+		path = filepath.Join(path, comp)
+		comps[i], err = r.realCaseBase(path)
+		if err != nil {
+			return "", err
+		}
+	}
+	return filepath.Join(comps...), nil
+}
+
+func (*basicRealCaserWindows) realCaseBase(path string) (string, error) {
+	p, err := syscall.UTF16PtrFromString(fixLongPath(path))
+	if err != nil {
+		return "", err
+	}
+	var fd syscall.Win32finddata
+	h, err := syscall.FindFirstFile(p, &fd)
+	if err != nil {
+		return "", err
+	}
+	syscall.FindClose(h)
+	return syscall.UTF16ToString(fd.FileName[:]), nil
+}
+
+func (r *basicRealCaserWindows) dropCache() {}