Greatly improve /proc/PID/stack by tracing the ebp frame chain.

I also added a generator cache to FileHandle. This way, multiple
reads to a generated file (i.e in a synthfs) can transparently
handle multiple calls to read() without the contents changing
between calls.

The cache is discarded at EOF (or when the FileHandle is destroyed.)

Userland/cat.cpp

@@ -15,7 +15,7 @@ int main(int argc, char** argv)
         return 1;
     }
     for (;;) {
-        char buf[4096];
+        char buf[1024];
         ssize_t nread = read(fd, buf, sizeof(buf));
         if (nread == 0)
             break;