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/config/config_test.go

@@ -22,7 +22,6 @@ import (
 	"github.com/d4l3k/messagediff"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
-	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
@@ -129,13 +128,6 @@ func TestDeviceConfig(t *testing.T) {
 			},
 		}
 
-		// The cachedFilesystem will have been resolved to an absolute path,
-		// depending on where the tests are running. Zero it out so we don't
-		// fail based on that.
-		for i := range cfg.Folders {
-			cfg.Folders[i].cachedFilesystem = nil
-		}
-
 		expectedDevices := []DeviceConfiguration{
 			{
 				DeviceID:        device1,
@@ -465,6 +457,7 @@ func TestFolderCheckPath(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	testFs := fs.NewFilesystem(fs.FilesystemTypeBasic, n)
 
 	err = os.MkdirAll(filepath.Join(n, "dir", ".stfolder"), os.FileMode(0777))
 	if err != nil {
@@ -489,7 +482,7 @@ func TestFolderCheckPath(t *testing.T) {
 		},
 	}
 
-	err = osutil.DebugSymlinkForTestsOnly(filepath.Join(n, "dir"), filepath.Join(n, "link"))
+	err = fs.DebugSymlinkForTestsOnly(testFs, testFs, "dir", "link")
 	if err == nil {
 		t.Log("running with symlink check")
 		testcases = append(testcases, struct {