linux nudge signal limitations: interrupts app system call; subsequent may be blocked

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 20, 2012 14:27:22_

we can't stop a nudge signal from interrupting an app system call.
to solve this we would have to resume each in a syscall-specific manner.

I hit this in the new nudge_ex test where the app sleeps.

on windows, nudge is processed in a new thread, so nudger can wait for that
to finish, so this problem does not exist.

also on linux only, if send two nudges in succession, the 2nd may come
while in the signal handler handling the first, in which case it will be
dropped on the floor (b/c not RT).

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=744_