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/osutil/osutil_test.go

@@ -62,7 +62,7 @@ func TestIsDeleted(t *testing.T) {
 		}
 	}
 	for _, n := range []string{"Dir", "File", "Del"} {
-		if err := osutil.DebugSymlinkForTestsOnly(filepath.Join(testFs.URI(), strings.ToLower(n)), filepath.Join(testFs.URI(), "linkTo"+n)); err != nil {
+		if err := fs.DebugSymlinkForTestsOnly(testFs, testFs, strings.ToLower(n), "linkTo"+n); err != nil {
 			if runtime.GOOS == "windows" {
 				t.Skip("Symlinks aren't working")
 			}