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

1 files changed, 10 insertions, 8 deletions

diff --git a/src/pkg/runtime/os_plan9.h b/src/pkg/runtime/os_plan9.h
index ea853dde8..57a2cafa7 100644
--- a/src/pkg/runtime/os_plan9.h
+++ b/src/pkg/runtime/os_plan9.h
@@ -79,12 +79,14 @@ struct Tos {
 	/* top of stack is here */
 };
 
-#define	NSIG	14	/* number of signals in runtime·SigTab array */
-#define	ERRMAX	128	/* max length of note string */
+enum {
+	NSIG = 14, /* number of signals in runtime·SigTab array */
+	ERRMAX = 128, /* max length of note string */
 
-/* Notes in runtime·sigtab that are handled by runtime·sigpanic. */
-#define	SIGRFAULT	2
-#define	SIGWFAULT	3
-#define	SIGINTDIV	4
-#define	SIGFLOAT	5
-#define	SIGTRAP		6
+	/* Notes in runtime·sigtab that are handled by runtime·sigpanic. */
+	SIGRFAULT = 2,
+	SIGWFAULT = 3,
+	SIGINTDIV = 4,
+	SIGFLOAT = 5,
+	SIGTRAP = 6,
+};