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

services/repository/files/file.go

@@ -129,7 +129,7 @@ func GetAuthorAndCommitterUsers(author, committer *IdentityOptions, doer *user_m
 // CleanUploadFileName Trims a filename and returns empty string if it is a .git directory
 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" {