runtime: reconvert sigqueue.goc from C to Go

The original conversion in CL 132090043 cut up
the function in an attempt to avoid converting most
of the code to Go. This contorts the control flow.

While debugging the onM signal stack bug,
I reconverted sigqueue.goc in its entirety.
This restores the original control flow, which is
much easier to understand.

The current conversion is correct, it's just complex
and will be hard to maintain. The new one is as
readable as the original code.

I uploaded sigqueue.goc as the initial copy of
sigqueue.go in the CL, so if you view the diffs
of sigqueue.go comparing against patch set 2 [sic]
it will show the actual starting point.

For example:
https://codereview.appspot.com/136160043/diff2/20001:60001/src/pkg/runtime/sigqueue.go

LGTM=dvyukov, iant
R=golang-codereviews, dvyukov, iant
CC=golang-codereviews, khr, r
https://codereview.appspot.com/136160043