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/model/testos_test.go

@@ -9,6 +9,8 @@ package model
 import (
 	"os"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/fs"
 )
 
 // fatal is the required common interface between *testing.B and *testing.T
@@ -28,6 +30,13 @@ func must(f fatal, err error) {
 	}
 }
 
+func mustRemove(f fatal, err error) {
+	f.Helper()
+	if err != nil && !fs.IsNotExist(err) {
+		f.Fatal(err)
+	}
+}
+
 func (f *fatalOs) Chmod(name string, mode os.FileMode) {
 	f.Helper()
 	must(f, os.Chmod(name, mode))