TBR: i#1984 sigaction: add missing error checks for sigaction params

Commit log for first patchset:
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i#1984 sigaction: add missing error checks for sigaction params

Adds missing error checks for invalid memory and other errors in sigaction
parameters, as we're often emulating the entire system call.

Enables the linux.sigaction_nosignals test.

Fixes #1984
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[bruening, 2016-12-11 05:16:59]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/0dcf7150ddf5fdc35ce9311c2c72985...

Final commit log: 
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i#1984 sigaction: add missing error checks for sigaction params

Adds missing error checks for invalid memory and other errors in sigaction
parameters, as we're often emulating the entire system call.

Enables the linux.sigaction_nosignals test.

Fixes #1984

Review-URL: https://codereview.appspot.com/319830043
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