runtime: do not handle signals before configuring handler

There was a small window during program initialization
where a signal could come in before the handling mechanisms
were set up to handle it.  Delay the signal-handler installation
until we're ready for the signals.

Fixes #3314.

R=golang-dev, dsymonds, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5833049

src/pkg/runtime/proc.c

@@ -734,6 +734,12 @@ runtime·mstart(void)
 	m->g0->sched.pc = (void*)-1;  // make sure it is never used
 	runtime·asminit();
 	runtime·minit();
+
+	// Install signal handlers; after minit so that minit can
+	// prepare the thread to be able to handle the signals.
+	if(m == &runtime·m0)
+		runtime·initsig();
+
 	schedule(nil);
 }
 
@@ -1161,7 +1167,7 @@ runtime·malg(int32 stacksize)
 {
 	G *newg;
 	byte *stk;
-	
+
 	if(StackTop < sizeof(Stktop)) {
 		runtime·printf("runtime: SizeofStktop=%d, should be >=%d\n", (int32)StackTop, (int32)sizeof(Stktop));
 		runtime·throw("runtime: bad stack.h");

src/pkg/runtime/runtime.c

@@ -119,7 +119,7 @@ void
 runtime·panicstring(int8 *s)
 {
 	Eface err;
-	
+
 	if(m->gcing) {
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic during gc");
@@ -189,7 +189,7 @@ runtime·goargs(void)
 {
 	String *s;
 	int32 i;
-	
+
 	// for windows implementation see "os" package
 	if(Windows)
 		return;
@@ -207,7 +207,7 @@ runtime·goenvs_unix(void)
 {
 	String *s;
 	int32 i, n;
-	
+
 	for(n=0; argv[argc+1+n] != 0; n++)
 		;
 
@@ -342,8 +342,6 @@ runtime·check(void)
 		runtime·throw("float32nan2");
 	if(!(i != i1))
 		runtime·throw("float32nan3");
-
-	runtime·initsig();
 }
 
 void