RFC: Lightweight synchronization mechanism for gnumach v3

[Agustina Arzille]

So, I kinda forgot to upload the newest version. Sorry about that.

Anyway, after some discussion, it was determined that the task's map had
to be locked between to calls to 'valid_access_p', when testing if the
addresses could be accessed with a certain protection. This prevents the kernel
from accessing invalid pointers if another thread deallocates the memory used
for gsync. Also, it's now the RPC entry points who are responsible for checking
that, instead of the function 'gsync_fill_key', which now simply initializes a 
key,
as the name implies.

The rest of the changes are merely cosmetic, with some polishing here and
there. The biggest change is the addition of read-locks as described above.

As usual, I'm open to any comments, criticism or advice you may have.

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