add support for signalfd

From bruen...@google.com on April 07, 2013 13:19:29

The linux kernel had some features added "recently" (relative to DR's signal handling implementation :)), in 2007 in the 2.6.22 kernel, that DR needs to handle because DR emulates that part of the kernel. signalfd is one of them. Today, DR will record the pending signal and just let the signalfd query block.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1138