Fix race condition between signal handler and main thread.

Fix race condition between signal handler and main thread.

The signal handler could unblock the main thread. The main thread
could delete the Process object the signal handler is holding a
pointer to. The signal handler could then close a file descriptor
it got from the deleted memory.

The whole process exit handling infrastructure is pretty fragile.
We should attempt to come up with something simpler.

R=sgjesse@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=3652

[Søren Gjesse, 2012-01-27 12:59:20]

lgtm

But as discussed offline we need to change the handling of terminating
processed. The global list of active processes need to be protected by locking
to avoid races. The process exit signal handler should only indicate that some
process is terminated. This indication should then picked up by some thread
which does the actual waitpid call and process list maintenance.