Return length from fwopen() file callback instead of zero.

The symptom is that syslog() fails to log anything but the "ident"
string if LOG_PERROR is specified to openlog(3) and the extensible
printf is in action.

For unclear, likely quaint historical reasons, syslog uses fwopen()
on a stack buffer, rather than using the more straightforward
and faster snprintf().

Along the way, fflush(3) is called, and since the callback writer
function returns zero instead of the length "written", __SERR
naturally gets set on the filedescriptor.

The extensible printf, in difference from the normal printf refuses
to output anything to an __SERR marked filedescriptor, and thus
the actual syslog message is supressed.

MFC:	after 2 weeks

lib/libc/gen/syslog.c

@@ -116,7 +116,7 @@ writehook(void *cookie, const char *buf, int len)
 		h->base += len;
 		h->left -= len;
 	}
-	return 0;
+	return len;
 }
 
 /*