Controlled Shutdown on Error (PVM/MPI)

This patch introduces a controlled shutdown functionality
for failing machines. A failing non-main machine will send
PP_FINISH to the main machine.
The main machine will (when failing or in a regular shutdown)
send PP_FINISH to all other machines, and expect PP_FINISH answers
which are counted to check (an earlier PP_FINISH will count towards
the sum). Therefore, the invariant is that all machines exchange
PP_FINISH messages with the main machine, in any order, before the
system shuts down.
Note also that nPEs is set to 0 at the end of MP_quit, to
avoid calling shutdownParallelSystem (and MP_quit) again.

This patch is for the MPI and PVM ways, code for the parcp way is
not ready yet.
Modify pack buffer failure, increase default

We should not ask for a bug report. In addition, this patch increases
the default size to 10 MB (the size should not be excessive, as the
RTS holds multiple internal structures of this size per PE).