SharedBuffer: Fix deadlock on destroy

We were locking the list of references, and then destroying the
reference, which made things go a little crazy.

It's more straightforward to just remove the per-reference lock: the
syscalls all have to lock the full list anyway, so let's just do that
and avoid the hassle.

While I'm at it, also move the SharedBuffer code out to its own file as it's
getting a little long and unwieldly, and Process.cpp is already huge.

Kernel/Makefile

@@ -6,6 +6,7 @@ KERNEL_OBJS = \
        StdLib.o \
        Arch/i386/CPU.o \
        Process.o \
+       SharedBuffer.o \
        Thread.o \
        Arch/i386/PIT.o \
        Devices/KeyboardDevice.o \