Introduce path Clean/Join helper functions (#23495)

Since #23493 has conflicts with latest commits, this PR is my proposal
for fixing #23371

Details are in the comments

And refactor the `modules/options` module, to make it always use
"filepath" to access local files.

Benefits:

* No need to do `util.CleanPath(strings.ReplaceAll(p, "\\", "/"))),
"/")` any more (not only one before)
* The function behaviors are clearly defined

routers/web/repo/editor.go

@@ -726,7 +726,7 @@ func UploadFilePost(ctx *context.Context) {
 
 func cleanUploadFileName(name string) string {
 	// Rebase the filename
-	name = strings.Trim(util.CleanPath(name), "/")
+	name = util.PathJoinRel(name)
 	// Git disallows any filenames to have a .git directory in them.
 	for _, part := range strings.Split(name, "/") {
 		if strings.ToLower(part) == ".git" {

routers/web/repo/lfs.go

@@ -207,7 +207,7 @@ func LFSLockFile(ctx *context.Context) {
 		ctx.Redirect(ctx.Repo.RepoLink + "/settings/lfs/locks")
 		return
 	}
-	lockPath = util.CleanPath(lockPath)
+	lockPath = util.PathJoinRel(lockPath)
 	if len(lockPath) == 0 {
 		ctx.Flash.Error(ctx.Tr("repo.settings.lfs_invalid_locking_path", originalPath))
 		ctx.Redirect(ctx.Repo.RepoLink + "/settings/lfs/locks")