Before this change, we have the kernel's sigaction in the uapi headers,
and our own sigaction in <bits/signal_types.h> and we rely on callers
making sure to use `#define` to move the kernel type out of the way if
they include a uapi header directly. This is obviously error-prone and
undesireable, and not what we usually do now.
What we _usually_ do now is use the header scrubber's ability to replace
a struct definition with a `#include <bits/STRUCT.h>`, but that doesn't
work here because struct sigaction relies on a lot of other types,
some of which also come from uapi headers.
So instead use our second best trick, which is to "move the kernel struct
out of the way" at header scrubbing time instead. This means that someone
who does `#include <linux/signal.h>` or `#include <asm/signal.h>` won't
get `struct sigaction` (they'll only have `struct __kernel_sigaction`
instead), but it does mean that they can't get two incompatible
definitions if they include a uapi header both directly and indirectly.
So although this doesn't do what I'd set out to do, it's still an
improvement in some cases, and it's our preferred idiom in most cases
anyway. (I'll come back once this is in to tidy up the two other kernel
structs where we're still using the deprecated "rename out of the way
using #define" trick, but this change is already hairy enough, and
there's a possibility it will break code that didn't care that it was
getting the kernel `struct sigaction` rather than the userspace one.)
Bug: http://b/236042740
Test: treehugger
Change-Id: Icff50e330c09c587e8f77ba0fb7cffffd9c3b708
df53b16fca