PATCH: Fix gsync module for remote tasks

[Agustina Arzille]

Hello, everyone.

Here's a patch that fixes gsync so that it doesn't panic when used by 
remote tasks.

Noteworthy changes:

- The type 'gsync_key' has been made into a union of several types in
  order to clearly denote what kind of address is being used.

- Sleeping locks are now being used to serialize access to gsync's 
buckets.
  The implementation can be found in the files kern/kmutex.{h,c}
  Note that it's a basic mutex; it doesn't support readers and writers
  separatedly (it's fully interruptible, though).

- The base function 'thread_wakeup_prim' was slightly modified so that
  it returns a boolean value, indicating whether any thread was 
scheduled
  to run or not. Callers are free to ignore its return value.

- An utility macro was added so that the reference count for VM objects
  can be incremented while holding the interlock.

- Most importantly, the gsync module has been reworked so that it now 
works
  with remote tasks, creating temporary mappings in the kernel's map for
  such special cases.

gsync.diff
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