rename of mode/context switching functions

- this patch only renames schedcheck() to switch_to_user(),
  cycles_accounting_stop() to context_stop() and restart() to
  +restore_user_context()

- the motivation is that since the introduction of schedcheck() it has
  been abused for many things. It deserves a better name.  It should
  express the fact that from the moment we call the function we are in
  the process of switching to user.

- cycles_accounting_stop() was originally a single purpose function.
  As this function is called at were convenient places it is used in
  for other things too, e.g. (un)locking the kernel. Thus it deserves
  a better name too.

- using the old name, restart() does not call schedcheck(), however
  calls to restart are replaced by calls to schedcheck()
  [switch_to_user] and it calls restart() [restore_user_context]

kernel/system.c

@@ -130,7 +130,7 @@ PUBLIC void kernel_call(message *m_user, struct proc * caller)
   caller->p_delivermsg_vir = (vir_bytes) m_user;
   /*
    * the ldt and cr3 of the caller process is loaded because it just've trapped
-   * into the kernel or was already set in schedcheck() before we resume
+   * into the kernel or was already set in switch_to_user() before we resume
    * execution of an interrupted kernel call
    */
   if (copy_msg_from_user(caller, m_user, &msg) == 0) {