handle pre-thread-init and post-exit signals

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on February 18, 2009 14:38:22_

with NPTL (POSIX) a SYS_kill signal is delivered to a single thread chosen
arbitrarily among those that aren't blocking it (each thread has its own
signal mask). this means a thread can receive a signal during our thread
initialization, before we've set up its dcontext and other structures, in
which case we die.

we also have problems when a signal comes in during process exit after
we've removed our handler: the app then dies ungracefully. technically
that could happen natively too but is much less likely since it exits more
quickly. this is less of an issue for release build and is more of an
annoyance for itimer testing.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=26_