runtime: write sigsetstack for Darwin, fix sigaction arg

runtime: write sigsetstack for Darwin, fix sigaction arg

It turns out that the second argument for sigaction on Darwin has a
different type than the first argument.  The second argument is the user
visible sigaction struct, and does not have the sa_tramp field.

I base this on
  http://www.opensource.apple.com/source/Libc/Libc-1081.1.3/sys/sigaction.c
not to mention actual testing.

While I was at it I removed a useless memclr in setsig, a relic of the C
code.

This CL is Darwin-specific changes.  The tests for this CL are in
https://golang.org/cl/17903 .

Change-Id: I61fe305c72311df6a589b49ad7b6e49b6960ca24
Reviewed-on: https://go-review.googlesource.com/18015
Reviewed-by: David Crawshaw <crawshaw@golang.org>