tree-wide: get rid of selinux_context_t (#3732)

https://github.com/SELinuxProject/selinux/commit/9eb9c9327563014ad6a807814e7975424642d5b9
deprecated selinux_context_t. Replace with a simple char* everywhere.

Alternative fix for #3719.

src/nspawn/nspawn.c

@@ -2642,7 +2642,7 @@ static int inner_child(
 
 #ifdef HAVE_SELINUX
         if (arg_selinux_context)
-                if (setexeccon((security_context_t) arg_selinux_context) < 0)
+                if (setexeccon(arg_selinux_context) < 0)
                         return log_error_errno(errno, "setexeccon(\"%s\") failed: %m", arg_selinux_context);
 #endif