ksud: refine su and make it compitable with MagiskSU

Supported features:

1.  --mount-master, -M, -mm which would make the command run in global mount namespace.
2. - user to switch a specific user's shell.
3. -v, -V to print version code and name.

fix #330 #306 #305 #32

userspace/ksud/src/cli.rs

@@ -141,7 +141,7 @@ pub fn run() -> Result<()> {
     // the kernel executes su with argv[0] = "su" and replace it with us
     let arg0 = std::env::args().next().unwrap_or_default();
     if arg0 == "su" || arg0 == "/system/bin/su" {
-        return crate::ksu::grant_root();
+        return crate::ksu::root_shell();
     }
 
     let cli = Args::parse();