Switch default disposition of realtime signals

Summary:
Realtime signals generally do not represent an error condition in an application but are more
like a regular means of IPC. As such, we shouldn't interrupt an application whenever it recieves
one. If any application will use these signals, it will probably use them a lot, rendering it's
debugging tiresome if we stopped at every signal. Furthermore, these signals are likely to be used
in a low level library, and the programmer may not even be aware of their presence.

For these reasons, I am switching the default disposition of realtime signals on all supported
platforms (i.e. Linux and Freebsd) to no-stop, no-notify. Any user still wishing to receive these
signals can always change the default to suit his needs.

Reviewers: ovyalov, emaste

Subscribers: lldb-commits, emaste

Differential Revision: http://reviews.llvm.org/D12795

llvm-svn: 247537