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kernel: return error from sysrfork instead of waiting and retrying

The old strategy of wait and retry doesnt seem to
work very well as it keeps all the forking parents
stuck waiting in the kernel worsening the situation.

The idea with this change is to have rfork() return
error quickly; and without whining; as most callers
would just react with a sysfatal() which might be
better for surviving this.

--- a/sys/src/9/port/pgrp.c

+++ b/sys/src/9/port/pgrp.c

@@ -268,7 +268,7 @@

 	char *p;

 	if(up == nil)

-		panic("resrcwait");

+		panic("resrcwait: %s", reason);

 	p = up->psstate;

 	if(reason != nil) {

--- a/sys/src/9/port/proc.c

+++ b/sys/src/9/port/proc.c

@@ -630,19 +630,13 @@

 Proc*

 newproc(void)

 {

-	char msg[64];

 	Proc *p;

 	lock(&procalloc);

-	for(;;) {

-		if((p = procalloc.free) != nil)

-			break;

-

-		snprint(msg, sizeof msg, "no procs; %s forking",

-			up != nil ? up->text: "kernel");

+	p = procalloc.free;

+	if(p == nil){

 		unlock(&procalloc);

-		resrcwait(msg);

-		lock(&procalloc);

+		return nil;

 	}

 	procalloc.free = p->qnext;

 	p->qnext = nil;

@@ -1409,7 +1403,8 @@

 	static Pgrp *kpgrp;

 	Proc *p;

-	p = newproc();

+	while((p = newproc()) == nil)

+		resrcwait("no procs for kproc");

 	qlock(&p->debug);

 	if(up != nil){

--- a/sys/src/9/port/sysproc.c

+++ b/sys/src/9/port/sysproc.c

@@ -84,7 +84,8 @@

 		return 0;

 	}

-	p = newproc();

+	if((p = newproc()) == nil)

+		error("no procs");

 	qlock(&p->debug);