[libj6] Update how init args are passed and used

Now the init args are a linked list - this also means ld.so can use its
own plus those of the program (eg, SLP and VFS handles). __init_libj6
now adds the head of the list to its global init_args structure, and the
j6_find_init_handle function can be used to find a handle in those args
for a given proto.

This fixes situations like the logger using the wrong mailbox for the
service locator and never finding the uart driver.

src/user/srv.init/main.cpp

@@ -75,7 +75,7 @@ main(int argc, const char **argv, const char **env)
         return s;
 
     const j6_init_args *initp = j6_get_init_args();
-    uintptr_t modules_addr = initp->args[0];
+    uintptr_t modules_addr = initp->argv[0];
 
     std::vector<const module*> mods;
     load_modules(modules_addr, sys, 0, mods);